Solar Powered Rainwater Harvesting Systems

1 Introduction

1.1 Solar-Powered Rainwater Harvesting System: Background

Consideration for Solar Powered Rainwater Harvesting System: South Africa has an estimated population of 54 million people, with an estimated 40% of this population living in rural settlements1 (MIT, 2016) (Stats SA, 2016). 74% of the people living in these rural areas rely solely upon groundwater, accounting for only 9% of the total water in use in South Africa due to a lack of water infrastructure (MIT, 2016). 19% of the rural population has no clean water supply, and 26% of schools and 45% of clinics have no water access (MIT, 2016).

Rainwater is a clean source of water that can be harvested to solve the need for potable water in rural areas (About, Inc., 2017). Rainwater is considered a clean water source due to its lack of contaminants and hard metals. The rainwater can become acidic depending on the location (EPA, 2017). Though the acidic part of rainwater is bad for metals and machinery, it is suitable for human consumption if filtered properly. The rainwater is polluted further by flowing from the catchment areas. Pollutants in these catchment areas include leaves, dust particles and droppings of various animals. Due to these pollutants, it is necessary to filter and purify the water according to the SANs 241-2015 standards for the water to be classified as potable water (HarvestH2o, 2017).

Rural areas are often found in areas of the country where there is little to no infrastructure in terms of water and electricity supply (ETU, 2017). To overcome this, a system design must be self-powered and automated.

1.2 Problem Statement

Investigate the possibility that a rainwater harvesting system can provide reliable potable water for a rural household. A system design that does not require external power and is simplistic in operation is needed. Test whether the addition of a slow sand filter in such a system design is a viable solution.

1.3 Objective of Solar-Powered Rainwater Harvesting System

The research project consists of a detailed literature study on relevant information. System diagrams were designed based on this literature study. The necessary design calculations were done to ensure these designs could function. A simulation was performed to test whether water received from rainwater
harvesting can fulfil potable water requirements for a rural household. Two different filtration techniques were chosen to test which technique would allow the overall system design to be solar-powered and automated. A water quality test was done to ensure the filtration technique chosen will meet the SANS 241-20152 standards for human consumption.

2 Solar Powered Rainwater Harvesting System: A Literature Survey

2.1 Introduction

A better understanding of rainwater harvesting is essential before any design or experimental procedures can be completed. Information about the required design stages will be investigated. Breaking a rainwater harvesting system into its separate stages will allow a better understanding of the type of components, filtration techniques, and pumping required at certain stages. All this research will aid in designing a self-powered rainwater harvesting system that will deliver potable water to a small household in a rural area.

2.2 Rainwater Harvesting

Rainwater harvesting is not a new concept, but because municipal water has been plentiful and of good quality, the modern generation has forgotten that many of our forefathers only had rainwater as a source of drinking water. We now have to re-discover the process of harvesting rainwater and apply modern technology and sophistication to supplement the dwindling supply of water (Tizagenix, 2017).

Harvesting rainwater is a very simple process and requires minimal effort. Once the fundamental components are installed, the weather will take care of the rest. Rainwater is usually captured by any roofed area, i.e. the roof of a house (Tizagenix, 2017). After the rain is collected on the roof, it flows through the following process steps:

1. Good primary rainwater filtration
2. Calming inlets
3. Floating suctions
4. Surface skimming overflows in the tanks
5. Post Filtration

These process steps need to be followed to design a successful rainwater harvesting system. Below is a description of the components used in the experimental rainwater harvesting system.

2.2.1 Components Used in Rainwater Harvesting Systems

2.2.1.1 Pre-filtration Wisy Filter

The WISY filter, as seen in Figure 2: Schematic drawing of WISY filter operation, is a primary filter used in rainwater harvesting systems. It filters away particles larger than 28 microns, ensuring that only clean filtered water enters the storage tank. Removing large particles will increase the tank’s lifespan and stop biomatter growth inside the tank. The WISY filter operates at 95% efficiency, meaning that only 5% of water passed through the filter is lost through the flush line.

2.2.1.2 Calming inlets Inlet

The WISY filter, as seen in Figure 2: Schematic drawing of WISY filter operation, is a primary filter used in rainwater harvesting systems. It filters away particles larger than 28 microns, ensuring that only clean filtered water enters the storage tank. Removing large particles will increase the tank’s lifespan and stop biomatter growth inside it. The WISY filter operates at 95% efficiency, meaning that only 5% of water passed through the filter is lost through the flush line.

2.2.1.2 Calming Inlets

Fine sediment carried in by the inflowing rainwater settles on the tank’s floor. This layer harbours microbes that provide a natural treatment to the water inside the tank; thus, it is referred to as the bio-layer. A calming inlet, as seen in Figure 3: Schematic drawing of Calming inlet, slows the incoming water and diverts it away from the bio-layer, thus preventing the inflowing water from disturbing the bio-layer. The calming inlet is placed at the bottom of the tank.

2.2.1.3 Floating Suction

The floating suction sits just below the water’s surface and will draw water from the top level. Figure 4: Description of a floating suction illustrates how it is placed inside a tank and how it looks. The top layer of water is the oldest and cleanest water in the tanks. It has the lowest concentration of entrained particulates and has had the longest ‘treatment’ time with healthy natural microbes.

2.2.1.4 Skimming Overflows

Some of the debris and contaminants float to the surface, forming a floating debris layer. Debris, such as pollen and oil-based matter, form a floating layer on the water surface. This builds up over time and eventually insulates the water from the air. This prevents oxygen transfer into the water, reducing the amount of aerobic microbial action. The overflows are designed so that the floating debris will get discharged out of the tank when the tanks overflow. Figure 5: Filter component used for Skimming overflow shows a typical skimming overflow that will be installed inside water tanks.

2.3 Post-Filtration of Solar-Powered Rainwater Harvesting System

After the rainwater has been harvested and collected in a storage tank, it needs to pass through a post-filtration stage to ensure it is safe for drinking. This experimental study will use a slow sand filter for its post-filtration stage. A slow sand filter was selected because it does not require a pump to force the water through the filter, as it is run by gravitational force (SSWM, 2017).

The slow sand filter will be tested and compared to current modern filtration techniques to determine if it is a viable solution to the post-filtration stage of rainwater harvesting.

2.3.1 Slow Sand Filtration

2.3.1.1 History

Slow sand filtration has been an effective water treatment process for preventing the spread of gastrointestinal diseases for over 150 years, having been used first in Great Britain and later in other European countries (American Water Work Association, 1991).

2.3.1.2 Previous Uses and Designs

Slow, sad filters were mostly used to filter water for large settlements before the Industrial Revolution. These filters were designed and built on a large scale and had to filter water for entire towns and cities (American Water Work Association, 1991). India launched a project to design and build slow sand filters on a small scale for low-income areas to use to filter their drinking water. These filters had several design flaws. People had to carry water to where the filters were located and could only filter a bucket of water at a time. The filters were often not operated properly because of the lack of understanding, which led to them running their sand beds dry and not functioning (Bio Sand Filter Organization, 2004). The Indian Slow sand filter can be seen below in Figure 6.

2.3.1.3 Structure

Traditionally, a slow sand filter consists of a filter vessel, namely a concrete box, the sand bed, and the required inlets and outlets for the water to flow. A typical design for a slow sand filter can be viewed in Figure 7 below.

2.3.1.4 Sand Bed

The design and layout of the sand bed is depicted in Figure 8: Sandbed design below:

2.3.1.5 Process

The basic principle of the process is very simple. Contaminated fresh water flows through a layer of sand, where it gets physically filtered and biologically treated. Hereby, both sediments and pathogens are removed. This process is based on the ability of organisms to remove pathogens. The top layers of the sand become biologically active by establishing a microbial community on the top layer of the sand substrate, also referred to as schmutzdecke3. These microbes usually come from the source water and establish a community within a few days. The fine sand and slow filtration rate facilitate the establishment of this microbial community. Most of the community are predatory bacteria that feed on water-borne microbes passing through the filter (American Water Work Association, 1991).

2.3.1.6 Advantages (SSWM, 2017)

• Very effective removal of bacteria, viruses, protozoa, turbidity and heavy metals in contaminated freshwater.
• The design is simple and highly self-help compatible: construction, operation, and maintenance only require basic skills and knowledge and minimal effort.
• If constructed with gravity flow only, no (electrical) pumps are required.
• Local materials can be used for construction.
• High reliability and ability to withstand fluctuations in water quality.
• No necessity for the application of chemicals.
• Easy to install in rural, semi-urban and remote areas, Simplicity of design and operation.
• Long lifespan (estimated >10 years)

2.3.1.7 Disadvantages (SSWM, 2017)

• Minimal quality and constant freshwater flow required: turbidity (<10-20 NTU) and low algae contamination. Otherwise, pre-treatment may be necessary.
• Cold temperatures lower the efficiency of the process due to decreased biological activity.
• Loss of productivity during the relatively long filter skimming and ripening periods.
• Regular maintenance is essential; some basic equipment or ready-made test kits are required to monitor physical and chemical parameters.
• There is a possible need for changes in attitude (the belief that water that flows through a green and slimy filter is safe to drink without the application of chemicals). Chemical compounds (e.g., fluorine) are not removed.
• Natural organic matter and other DBP precursors not removed (maybe formed if chlorine is applied for final disinfection)
• It may require electricity.
• The requirement is a large land area, large quantities of filter media, manual labour for cleaning, and a low filtration rate.

2.3.1.8 Effectiveness

Slow sand filters consistently demonstrate their effectiveness in removing suspended particles with effluent turbidities below 1.0 nephelometric turbidity units (NTU), achieving 90% to 99% reductions in bacteria and viruses and providing virtually complete Giardia lamblia cyst and Cryptosporidium oocyst removal (Tech Brief, 2014). A more detailed look at the effectiveness of the slow sand filter can be viewed in Table 1 below:

2.3.1.9 Important Facts about Slow Sand Filter (ITACANET, 2005)

• The sandbed must be kept wet at all times. If the sandbed runs dry, the biolayer will die off.
• As defined by the hydraulic loading rate (HLR), the flow-through sand bed should be kept as low as possible, preferably between 0.1 – 0.4m/h.
• The biolayer takes 3-7 days to become fully active and effectively filter the water.
• Over time, the top layer of the sand bed will become too densely populated by the biolayer. This will cause the sand layer to become blocked and not allow water to pass through. The top layer of sand then needs to be scraped off and cleaned.

2.3.2 Forced Filtration

2.3.2.1 Definition

In potable systems, the following post-filtration steps are implemented (GERM Africa, 2017):

1. Fines filtration
2. Activated carbon and KDF treatment
3.  Ultra-Violet Sterilization

2.3.2.2 Fines Filtration

Various methods are used to remove fine particulates from the water. We generally filter all particulates larger than 1 micron from the water. In some domestic installations, a back-washable sand filter that removes particulates larger than 80 microns is the first step, followed by a 20-micron pleated cartridge filter and a 1-micron pleated cartridge filter.

2.3.2.3 Activated Carbon and KDF

The activated carbon and KDF units treat the water for any residual colour and odour. They also remove chlorides and heavy metals.

2.3.2.4 Ultra-Violet Sterilization

The UV sterilizer kills 99.9% of all microbes that might still be present in the water.

The above steps are scalable and can be applied to large industrial-scale installations and small domestic systems.

Forced Filtration consists of a multistage filtration process to ensure the best quality of potable water. A typical layout of such a system can be viewed in Figure 9: Schematic depicting a forced filtration setup. The forced filtration setup comes at a large additional cost and power usage. A large external pump is required to deliver the required pressure for the filters to work, and the UV light requires additional electricity. The filters and UV lightbulb have a set lifetime and must be replaced at the end of each lifetime. This escalated the cost of such a system dramatically (GERM Africa, 2017).

2.4 Water Usage

The typical household (2 adults and three children) in South Africa uses about 250 litres of water a day. That amounts to 7,500 litres a month, already more than the monthly free water allowance every household receives from the municipality. The typical South African household consisting of 3 children and 2 parents would then use an average of 37,500 litres of water a month (Aquarista, 2017).

The average water consumption can then be divided into various fields depending on the water used. For the purpose of this research, project interest is only shown where potable water is needed, i.e., for bathroom and kitchen uses, as shown in Figure 10: Average Family Water Consumption.

2.5 Solar Power

For the system to be fully automated and provide potable water for the household inside the house, an element of pumping will be required. This pump will require a power source, and due to the lack of electrical infrastructure in rural areas, it has been decided to make the system solar-powered.

2.5.1 Factors Influencing Solar Power

There is no such thing as a perfect technology. Research reveals the different factors that can affect the efficiency of solar panel mounting systems. Some of these factors have been studied to either increase or decrease the power production from the three types of mountings such as sun intensity, cloud cover, relative humidity, and heat buildup. When the sun is at its peak (intense), during midday, most solar energy is collected; therefore, there is an increase in the power output. Cloudy days contribute to the decrease in sunlight collection effectiveness since clouds reflect some of the sun’s rays and limit the amount of sun absorption by the panels. During summer days, when the temperature is at its highest and heat is built up quickly, the solar power output is reduced by 10% to 25% for the reason that too much heat increases the conductivity of semiconductors, making the charges balance and reducing the magnitude of the electric field. In addition, if humidity penetrates the solar panel frame, this can reduce the panel’s performance, producing less power and, worse, can permanently deteriorate the modules’ performance (Gordo, Khalaf, & Strangeowl, 2015).

2.5.2 Solar-Powered Rainwater Harvesting System: Effective Solar Energy Available

Figure 10 below is a solar map of South Africa depicting the solar energy available in the different areas of the country.

Figure 11: Solar map of South Africa indicates that coastal areas have less available solar energy than inland areas. The coastal areas offer enough to power a small-scale system that does not require a lot of power and does not require solar energy every day. Therefore, a battery will be required to design a solar system close to the coast.

2.5.3 Solar Charge Controllers for Solar Powered Rainwater Harvesting System

A charge controller is a voltage and/or current regulator that keeps batteries from overcharging. It regulates the voltage and current from the solar panels to the battery. Most “12 volts” panels put out about 16 to 20 volts, so the batteries will be damaged from overcharging if there is no regulation. Most batteries need around 14 to 14.5 volts to get fully charged (Wind & Sun, 2017)

2.6 Conclusion

From the literature about rainwater harvesting, it is clear that to harvest and use rainwater, the appropriate steps have been taken. Ensuring all primary processes are in place made the system more efficient and reliable.

Post-filtration is essential for this system design, and the experimental test to follow will prove whether the literature about slow sand filters can be applied to a system design such as this one.

The information discussed above will be used to implement a functioning solar power rainwater harvesting system to deliver potable water to a rural household via a slow sand filter.

3 Solar Powered Rainwater Harvesting System: Flow diagrams

A rainwater harvesting system was designed following the correct process steps in the literature review. To design the system to be fully automated and powered by solar power, it was decided to use a slow sand filter in the post-filtration stage. A second system was designed using a more
conventional filtration technique, forced fines filtration, to see how the slow sand filtration technique would measure up to conventional techniques.

Both systems were designed using the same rainwater harvesting processes. The only difference is the postfiltration techniques.

Flow diagrams were drawn using AUTO CAD to give a clear idea of how the experimental setup would be constructed.

3.1 Flow Diagram for Slow Sand Filtration Setup

• Stage 1: Rain will be harvested from the 35sqm roof area. The water will flow down the installed gutter systems and pass through the Wisy 110mm inline filter. The discharge of this filter will enter the 1000l storage tank via a calming inlet as per design requirements. The flush line of the Wisy
  the filter will run out onto the concrete surface area and down the stormwater lines.
• Stage 2: The Rainwater will be stored in a 1000l Rotor tank. The tank will be fitted with a calming inlet and skimming overflow, as per the design requirements. The tank will be installed at a height above the slow sand filter, allowing the water to flow to the slow sand filter using gravity.

• Stage 3: The rainwater will enter the top of the slow sand filter via a calming inlet so as not to disturb the top layer of the Schmutzdecke. The flow of the rainwater will be regulated by an orifice installed at the entry point to the slow sand filter. This will ensure that the flow rate through the
  filter does not exceed the recommended 0.4m/h. The slow sand filter will be designed with the appropriate drain and overflow valves; it will also include an outlet system to capture water at the bottom of the filter. The filter media in the slow sand filter will consist of a layer of coarse, clean gravel. The gravel will support the sand layers above it. The sand layers will consist of a coarse layer of filter sand supporting the finer river sand above it.
• Stage 4: This stage consists of the ground-level storage tank. The filtered water will flow into the storage tank via a “gooseneck” plumbing feature, as seen in Figure 18. This configuration will allow water to flow into the storage tank but will not allow the sand bed to run dry. Running the sand bed dry will kill the Schmutzdecke.
• Stage 5: The solar panel will be installed on the roof of the building in an area with no shade and will face north to gather most of the sunlight in a day. The charge from the solar panel will run to a solar charger that will keep the 12V battery charged at all times. The 12V battery
  will supply power to the solar pump.
• Stage 6: The solar pump will pump the water up to a top-level storage tank. The pump uses a small amount of power but also delivers a low flow rate. An electronic float switch will be installed in the ground-level storage tank to ensure the pump does not run dry.
• Stage 7: The solar pump will pump the water to a top-level storage tank (roof height). The top-level tank will ensure enough pressure for the user inside the house via gravity when water is needed. This also allows for the use of a small solar pump that uses less power and delivers a smaller flow rate. A float valve will be installed in the top-level storage tank that will switch off the pump when the tank is filled.
• Stage 8: The user inside the house will be able to open a specific tap for potable drinking water. This tap will be used each day to simulate a household’s daily potable water consumption needs.

Figure 13 depicts a three-dimensional view of the layout for the proposed system. The figure shows that the 1000l storage tank is a height above the slow sand filter drum. This is due to the layout of the chosen location where the experimental procedure is to take place. This height difference is enough to allow the outlet from the storage tank to be the same height as the inlet to the slow sand filter. Therefore, there is no need to construct a stand for the storage tank to be placed on.

3.2 Flow Diagram for Forced Filtration Setup

• Stage 1: Rain will be harvested from the 35sqm roof area. The water will flow down the installed gutter systems and pass through the Wisy 110mm inline filter. The discharge of this filter will enter the 1000l storage tank via a calming inlet as per design requirements. The flush line of the Wisy
  filter will run out onto the concrete surface area and down the stormwater lines.
• Stage 2: The Rainwater will be stored in a 1000l Rotor tank. The tank will be fitted with a calming inlet and skimming overflow, as per the design requirements.
• Stage 3: Power for the system will be supplied by the 40W solar panel. The panel will be installed on the roof of the building in an area that is not shaded at any point in the day. The panel will be installed so that it faces north to get the best exposure to the sun. The solar panel will provide.
  The battery pack is charged to the solar charger, which keeps it charged at all times. An inverter converts the 12V DC current into a 220V AC current. The power supply is run to the UV light and the submersible pump.
• Stage 4: A submersible pump will pump the stored rainwater out of the tank and through the filtration set into the household. The submersible pump will be fitted with a pressure pump controller to turn on when the tap is opened inside the house. The pump will also have a float switch to protect it from running dry.

• Stage 5: The submersible pump will pump the rainwater through the filtration set. The filtration set consists of a 20-micron filter then a 5-micron filter. After the fines filtration stage, the water will pass through an active carbon KDF filter. From here, the remaining bacteria in the water will be
  removed by passing the water over an ultra-violet filter.
• Stage 6: The water will run straight from the filtration set into the household. The water can be accessed by opening the required tap for potable water.

3.3 System Flow Diagrams Discussion

The information about the two different system diagrams indicates that the slow sand filter design will be better suited for the application in a rural settlement environment. Due to its low energy requirements, low cost, easy maintenance, and simplicity, the slow sand filter design will be a preferable system design. In order to establish if the slow sand filter system design is effective, it will have to be tested against the conventional forced filtration method. Therefore, both systems will be manufactured and tested.

4 Solar Powered Rainwater Harvesting System: Viability of RWH System for Rural Household Potable Water Supply

A test needed to be performed to establish if harvesting rainwater can supply enough water to satisfy a rural household’s potable water needs. Rainfall is dependent on weather patterns and is always changing. To perform this test and take into account the ever-changing rain patterns, a Matlab Simulink model was created.

The Simulink model will use daily rainfall data from the last ten years in the Amanzimtoti area. This dataset provides a large enough data population to determine accurate mean values for daily rainfall. The dataset takes into account outliers, I.e., Freak storms or droughts, but will not take into account the global weather change over the last century. The dataset was gathered by a national weather station located in Amanzimtoti (SA Weather Service, 2017).

The Simulink model will create a forecast of the daily rainfall over a period of a year in the Amanzimtoti area. The forecast model can be viewed below in Figure 17: Forecast model for daily rainfall. The block diagram design of the Simulink model can be viewed in Appendix G.

Another valuable piece of information needed to complete the test is the average daily potable water consumption that will take place. The literature review shows that a family of 5 uses approximately 250L of water per day (Aquarista, 2017). This equates to 50L per person. Using Figure 10: Average Family water Consumption, the amount of potable water usage was calculated using only the water used in the bathroom and the kitchen, as the other water uses do not require the water to be of potable standards:

It should be noted that adding up all the rain throughout the year and dividing it by the amount of water usage will not give an accurate result as a number of variables need to be accounted for. These variables include the roof area that will be used for harvesting rainwater, the efficiency of the prefiltration method, the storage tank volume and the tank over-flow periods. The Simulink model allows inserting the values of the roof area, storage tank volume and daily water consumption. It uses the calculated forecast model with these variables to run the simulation. The results from the simulation show the tank level throughout the year. It also indicates the overall system efficiency, total water harvested and total water lost.

To run an experimental procedure, a small roofed building had to be selected for the location of the experiment. A backroom in the yard of Manna Hoogenboezem was selected as the experimental building. It yielded the same environment as rural area households with a roof area of 35 sqm and corrugated iron roof sheeting. The building lies in an area of densely populated trees and plant growth. Animal life, birds and monkeys are also present. It has one bedroom a small kitchen and a bathroom, ideally suited for one person to live there. Therefore, the chosen building correctly represents the intended rural household.

The following information was used to simulate the experimental procedure:

• Tank Volume 1000l
• Roof area 35sqm
• Daily water consumption of 20l (including safety factor of 1.3)
• Pre-filtration efficiency of 95%

The simulation used the information above and the forecasted rainfall model to predict the tank level throughout the year. The result can be viewed in figure 18:

Figure 18 above shows that harvesting rainwater can provide enough water to supply the potable water needed throughout most of the year. It is clear that in the winter months, with less rainfall, there will be a shortage. The supply will be sufficient if water management is implemented throughout these months.

To verify that the simulation is correct, actual rainfall data were recorded throughout the experimental timeframe (19th February 2017 – 11th May 2017). This data set could then graphically be represented against the predicted rainfall data for the same period. Figure 19 below shows the resultant comparison:

Figure 19: Predicted rainfall vs actual rainfall comparison shows that the actual rainfall in the experimental time frame follows the same pattern as the predicted rainfall. Figure 19 proves that the simulation model used is accurate and correct. From the predictions made by the simulation, it can be stated that harvesting rainwater will supply sufficient water to satisfy a rural household’s potable water needs.

5 Solar Powered Rainwater Harvesting System: Design Calculations

Before the systems in section 3 could be built, a number of design calculations had to be completed to ensure they would perform as required.

5.1 Slow Sand Filter Setup Calculations

5.1.1 Flow Rate through Sand Filter

As discussed in the literature review section 4.2.7, for the Schumtzdecke to be effective, the water passing through the sand filter needs to flow at a constant slow flow rate. Therefore, a hydraulic loading rate of 0.4m/h was chosen to design the sand filter. Equation [1] can calculate the required flow rate into the sand filter.

[1]

5.1.2 Orifice Calculations for Slow Sand Filter Design

Part of the system design’s automation process is to place the harvested rainwater storage tank at a height above the slow sand filter. This will allow gravity to feed the water into the slow sand filter. As seen in the literature study on slow sand filters, the flow through the filter must be controlled for the filter to work effectively. To keep the flow restricted below 19.63l/h, as calculated in section 4.1, it was decided to design and install an orifice. The operation of an orifice is depicted in Figure 20 below.

The flow rate through an orifice plate can be determined by the use of equation [2] as stated below (White, 2011):

[2]

Using the properties and dimensions listed in Table 2 below, the required pressure drop across the orifice plate could be determined:

The static pressure before the orifice is calculated using equation [3] derived from Bernoulli’s equation:

[3]

The pressure after the orifice plate is calculated using the required flow rate into the slow sand filter and with equation [4]:

[4]

The discharge coefficient depends on the Reynolds number of the flow through the orifice plate. The Reynolds number is determined using equations [5] and [6] as seen below:

[5]

[6]

The results from [6] were used together with Figure 21: Discharge Coefficient for thin plate orifice with D: 1/2D taps to determine the discharge coefficient for different diameter orifice plates. As the size of the orifice increased, the discharge coefficients ranged from 0.605 to 0.62.

These values, together with the known required flow rate, were used in an Excel spreadsheet to determine the required orifice plate diameter.

The Excel spreadsheet proved that the required orifice plate diameter is 3mm. Therefore, a 3mm diameter orifice was installed in the experimental setup.

Using the calculated 3mm diameter orifice and equation [2], a new spread was set up to determine the flow rate into the slow sand filter at the different levels of the storage tank. This was done to ensure the flow rate never exceeded the recommended rate of 19l/h and to determine the lowest flow rate that would flow into the slow sand filter. The resultant data can be viewed in Figure 22.

An experimental test was run to verify the calculations above. Water flowed from the tank through the installed orifice into a 20l bucket for an hour. This procedure was followed for three different tank levels. The water in the bucket was measured and recorded after the test. The data can be viewed in Figure 23 below:

As seen in Figure 22, the calculated flow rate never exceeds the recommended 19.63l/h. This information is verified by the experimental data in Figure 23, which shows that the experimental flow rate follows the same curve as the calculated flow and does not exceed the recommended value either. Figure 24 shows the chosen orifice plate to be installed.

5.1.3 Pump Selection for Solar-Powered Rainwater Harvesting System

The system design aims to make the system as automated as possible, but a pumping system will be required to supply water to the household.
The goal for the SSF system layout is to use as little electrical energy as possible. However, potable drinking water still needs to be delivered to the household at pressure. To accomplish this and use less energy, the proposed layout uses a small pump that pumps the water at a low flow rate to a storage tank at the roof height of the house. The water is then supplied to the household, using gravity to supply the pressure.

To solve for the system requirements, Bernoulli’s equation is used as seen in equation [7] below:

[7]

Where hp is the head increase across the pump. But since P1=P2 and V1=V2=0, solve for the pump head:

[8]

There is no specified flow rate for the system design, but in order to save power, the flow rate should be specified as low as possible while still being able to pump the maximum daily water consumption. Therefore, the maximum required flow rate will be 200l/day.

[9]

Now list and sum the minor loss coefficients:

Calculate Reynolds numbed and pipe friction factor using equation [6]:

Therefore, the flow in the pipe is the laminar flow:

[10]

A system requirement curve was calculated for different flow rates using the above calculations.

Figure 25: The system requirement curve the pump must satisfy for the design to function. Figure 26: The pump curve shows the pump curve for the 12V solar pump Xc Trading Model XC-SP-002-12C. Figure 26 indicates that the chosen pump will deliver the required head at the required flow rate as the supply points exceed the required points. Therefore, the chosen pump will function in this design.

To pump the water at the required rate and height, the pump must displace power to the water of:

[11]

This is the pump’s theoretical power requirement. Experimental pressure and flow rate readings were taken; see Table 5 for results. This data can then be used to calculate the pump’s actual power requirement.

Therefore, the actual power requirement from the pump is calculated to be:

[12]

5.1.4 Power Supply Calculations for Solar Powered Rainwater Harvesting System

The pump is the only component that requires electrical energy to operate. The pump’s power will be supplied by a 12V battery that will be kept charged by a solar panel. Calculations were done to verify how effectively the chosen components would power the system.

5.1.4.1 Components Specification

The components include a solar pump, a 12V battery and a 40W solar panel. These components were selected as they were available for use to complete the experimental procedure and did not have to be purchased. The specifications of these components can be found in Table 6 below. The specifications of the components were listed as designed by the manufacturing companies.

The pump will run for three 5-minute cycles per day to satisfy the required 20l per day water usage, which adds up to 0.25 hours of run time.

[13]

With the installed solar charge controller, the battery can only drain 50% of its capacity.

[14]

7 days run time is enough to keep the pump running throughout cloudy and rainy days when no solar charge is available. When the sun does come out, the battery can be charged:

[15]

Considering the losses in the solar panel and the sun factors, the time required to charge the battery is to be 5 hours. As there are only 6 hours of effective sunlight in an average day, it is safe to assume the system will only require one sunny day to recharge and be ready for the next 7 days.

5.1.5 Head Loss through Sand Bed

As discussed in the literature, the slow sand filtration process depends on the biolayer that grows on the top layer of the sand bed. Over time, this layer will become too densely populated and block the flow of water through the filter. At this point, the user will have to scrape off the top layer of sand and wash it (American Water Work Association, 1991).

Layer piezometers have been installed in the experimental setup so the user knows when to clean the sand. These piezometers will indicate the head loss across the sand bed. The maximum allowable head loss can be calculated using Darcy’s law as seen in equation [16] below:

[16]

Figure 27: Head loss through porous medium indicates how equation [16] was implemented on the experimental setup.

5.1.5.1 The Hydraulic Conductivity, K, for different sands and gravels are depicted in Table 5 below:

The hydraulic conductivity, K, is a constant for a given porous medium. A very porous medium with little resistance to flow will have a high value for K, while a tightly packed medium with high resistance to flow will have a low value for K.

The slow sand filter is specified to operate at a hydraulic loading rate of 0.1 – 0.4 m/h. Therefore, if the HLR is below 0.1m/h, the sand bed can be declared blocked and needs cleaning.

Therefore, using Equation 12, figure 4, and Table 5, the maximum allowable head loss through the sand bed can be calculated as follows.

Therefore, it can be stated as the Schmutzdecke becomes more populated, the top layer of sand becomes more blocked. This will decrease the hydraulic conductivity of the sand. If the same flow rate is required through the filter, the head loss across the filter bed will increase. This head loss can be viewed on the piezometers installed on the sand filter. The head-loss required for the user to know when to clean the sand bed will differ from system to system as the sand will vary. An installed system should be monitored and checked regularly, for when the flow rate out of the sand filter reaches as low as 4.91e-03m3/s, the filter bed needs to be cleaned. At this point, a mark can be made on the piezometers so that they can be monitored for that particular sand filter in the future.

5.2 Forced Filtration Setup Calculations

A filtration skid was provided to complete the experimental procedure for the forced filtration system setup. Forced fines filtration systems are common in households around the world. They all operate under the same principle as described in the literature study.

5.2.1 System Requirement Curve

The pressure drop across the filtration system was measured by reading the pressure at the pump outlet and after the entire filtration skid. As the filtration skid is located at the point of water usage, the frictional energy loss was assumed to be minimal. Therefore, the pressure drop across the filtration system can be assumed to be the system requirement curve, which can be seen in Figure 28.

It should be noted as the filters become dirty over time, they increase the resistance of the filtration system. Therefore, it is important that the chosen pump can deliver the required flow rate through the dirty filters.

The chosen pump for Tizagenix small-scale filtration skids is a Shimge QT Peripheral Pump .37kW. The pump curve for the chosen pump can be viewed in Figure 29 on the QT37 line. Figure 29 indicates that at a flow rate of 50l/min, the .37kW pump will provide enough head to supply the filtration skid even with dirty filters.

5.2.2 Power supply

To pump the water at the required rate and height, the pump must displace power to the water of:

[11]

The pump is not the only component that requires electrical energy to function. An 11W UV light bulb also needs power.

That leaves the total energy requirement of the system to be 112.98W.

The system will run for three 10-second cycles daily to satisfy the required 20 litres of water daily. This adds up to 0.00833 hours of run time.

[13]

With the installed solar charge controller, the battery can only drain 50% of its capacity.

[14]

2 days run time is not enough to keep the pump running throughout cloudy and rainy days when there is no solar charge available. A bigger battery system will be needed to keep the system operating longer without charge.

Due to the short power supply time and the fact that the pump and UV light require 220V AC current, it was decided to plug the system into the household’s power supply for the experiment. This was done to avoid buying additional batteries and inverters.

The design calculations have proven that the forced filtration can be solar-powered but requires 100 times more energy than the slow sand filtration setup.

6 Solar Powered Rainwater Harvesting System: Experimental Procedure Components

After the designs of each system were verified with the necessary calculations, the experimental setup of each design could be assembled.

6.1 Building Procedure for SSF Design

• All water storage tanks were cleaned and sterilized. The tanks used for the experimental procedure are shown below in Figure 30. According to the design drawings, holes were drilled in the correct locations for installing tank fittings.

• The correct guttering system was installed on the roof of the chosen building. The gutter network was then connected to a Wisy 110 inline filter. The discharge of the Wisy filter was routed to the 1000l storage tank. The flush line of the Wisy was routed down to the concrete floor, where the water would run into the stormwater drains. Photos of the gutter network and the connection to the Wizy inline filter can be viewed below in Figure 31.

• Figure 32 shows the correct fittings and installed drains as per the design drawings.

• Calming inlets and drainage were manufactured and installed as per Figure 33 below.

• The Sand filter was filled to the correct levels with the correct sand per design requirements. The first layer contained coarse gravel (38mm). The coarse gravel supports the sand bed so it does not drain away with the water. The gravel was washed before being inserted into the filter. A layer of filter sand was then added (38mm). This type of sand is predominantly used in rapid-rate sand filters and is a quality course. The final and thickest layer of river sand was then added to the filter (450mm). (ITACANET, 2005). Figure 34 shows photos of the sand and gravel used in the slow sand filter.

• All systems were interconnected using various valves and piping, as shown in Figure 35.

• The chosen orifice plate was installed at the inlet of the slow sand filter. A float valve (Figure 36) was installed inside the filter to prevent overflow.

• An afloat switch (Figure 37) was placed in the second storage tank and connected to the pump’s power source. This will ensure the pump does not run dry when the tank is empty.

• A float valve was installed in the top tank to ensure the tank does not overflow and wastewater. When the float valve shuts off, the cut-off pressure of the solar pump will be exceeded, and the pump will shut off. This will save electricity and ensure that the pump only activates when water is used.
• The 40W solar panel was installed on the building’s roof. It was wired up and connected to the solar charge controller. This will ensure the 12V battery is not overcharged or drained below 50% of its capacity.
• The battery and the pump are connected to the solar charge controller.

After the building procedure, rainwater was allowed to run into the sand filter and fill the sand bed. All valves were closed, and the water was allowed to stay in the filter for 4 days. This was done to give the bio-layer a chance to grow. After this initiation period, the system was allowed to run. 20l of water was drained from the top storage tank daily to simulate the daily water consumption. Section 7.2 will highlight the procedure followed to take water samples every week to test the system’s water quality.

Several errors were noticed throughout the system’s run time and had to be addressed.

6.1.1.1 Rectifications and Adjustments

After the first two weeks of run time for the system, it was noted that it was filtering the rainwater, and the bacteria count was decreasing. The 3rd week’s water sample result showed the water after the filtration process contained more bacteria than the pre-filtered water. Inspection of the system revealed that leaves and other bio-matter had managed to enter the storage tanks via the openings and doors. The system was drained and cleaned. The slow sand filter was isolated so as not to disrupt the Schumtzdecke. After resetting the system, the storage tanks were sealed off with plastic bags to ensure no bio-matter could enter again and influence the water quality. The system was allowed to run again.

After the water sample was taken on March 17th, the water quality started to worsen again. A good look at the experimental setup was done, and a number of errors were found that needed to be addressed. These errors included blocked gutters and incorrect sand sizes.

The area in which the experimental setup is located is surrounded by a dense population of trees, which simulate real-life circumstances. These trees cause the gutters to fill to the brim with leaves. As these leaves blocked the gutters, they lay in stagnant water. Therefore, the leaves caused less water to enter the system when it rained but also caused rotten water to enter the tank and contaminate the rest of the stored water.

The gutters in Figure 38 were cleaned, and the system was cleaned out and reset to get rid of the contaminated water.

After analyzing the experimental setup for possible flaws, it was noted that the sand in the top layer of the sand bed did not conform to the standards made clear in the literature. Therefore, the slow sand filter was operating and filtering the water but not effectively enough, as the top layer of sand particles was bigger than specified and did not allow the Schmutzdecke to form properly.

The sand bed was removed from the filter drum and washed, as seen in Figure 39 above. Plaster sand was considered a replacement top sand as it is much finer in particle size than river sand. However, plaster sand contains high amounts of clay and is not clean enough to work inside the slow sand filter.

The fine beach sand was then selected for the replacement in the filter box. The sand was properly washed and entered at the correct depths and specifications. The filter was filled with rainwater and left for 4 days for the Schmutzdecke to be re-established. The system was run as usual after the initiation period was over. Water samples were taken once a week and proved that the corrections made a difference as the samples returned zero bacteria count for the filtered water.

6.2 Building Procedure of Forced Filtration Design

• The forced filtration setup will use the same roofed structure, gutter network, prefiltration, and storage tank as the SSF setup.
• As per the design, water will be pumped out of the storage tank using the Shimge QT peripheral pump. Figure 40 depicts what the pump looks like. The pump’s power will come from a 220V wall socket as it requires an alternation current and, as calculated, will drain the solar power supply too quickly to function.

• A float switch will be connected to the pump’s power source. This will allow the pump to turn off when the water level in the storage tank is too low. The float switch can be viewed in Figure 41. The pump comes installed with its own pump controller. This means the pump starts when the tap is opened inside the house and water is required.

• The filtration was pre-ordered and manufactured. The pump was connected to the filtration unit inlet. The outlet was connected to the plumbing of the small building, allowing the tap inside to use the filtered rainwater. The installed filtration set can be viewed in Figure 42 below.

Once the forced filtration system was installed, it was tested. All components functioned according to the design. A water sample was taken of the post-filtered water, and the micro-slide proved that no bacteria were present in the water after the filtration. Thus, no further adjustments needed to be made to the system layout.

7 Solar Powered Rainwater Harvesting System: Data Recordings

7.1 Rainfall

Section 4 of the report proves that rainwater harvesting can provide enough water to satisfy the potable water needs of a small household. A simulation was performed to prove this fact, and it was verified by recording the actual rainfall over the experimental time frame from 19 February 2017 to 11 May 2017. The recorded actual rainfall can be viewed in section 4 under Figure 19: Predicted rainfall vs. actual rainfall comparison.

Rainfall was measured by a rain gauge on the same property as the experimental setup. No trees or buildings obstruct the rain gauge to ensure accurate data was recorded. The rain gauge was emptied every day, and the reading was recorded. The rain gauge can be viewed below in
Figure 44.

7.2 Water Samples for Slow Sand Filter Setup

Part of the objective of this research study was to determine if a slow sand filter can purify rainwater to potable standards. To test whether this is possible, water samples were taken. Sending a water sample to a lab and getting it analyzed is very expensive and takes time. Therefore, a basic water quality test was performed using Mikrocount Combi test slides, as seen in Figure 45, while the experimental setup was fine-tuned.

The quality test was taken once weekly for the duration of the experimental period. These samples are taken by dipping the test slide into the water and placing it back in the incubation chamber. The slides must be left to incubate for 48 hours. After 48 hours, the slide is removed, and data is read off the bacterial scale provided in the test kit, as seen in Figure 47. One side of the slide will show the bacterial count in the water, and the other will show the yeast and fungi count, as seen in Figure 46.

Samples will be taken from 3 different locations each time.

• Location 1 (Figure 48): This water sample is from the 1000l storage tank (pre-filtration). It is being taken to determine how many bacteria are present in the rainwater harvested off the roof.

FIGURE 47: LOCATION 1 WATER SAMPLE

• Location 2 (Figure 49): This water sample is from the second small storage tank on ground level (post-filtration). Its purpose is to determine how much bacteria is left in the water after passing through the filter.
• Location 3 (Figure 49): This water sample is stored at the top of the roof (post-filtration, after some time). This is to determine if the fresh filtered water will stay clean after being stored for several days; this will also show if there is any bacterial regrowth.

7.2.1 Data Recordings

As seen from Table 9, the water in the 1000l storage tank contains many bacteria and is, therefore, unsuitable for drinking water. As discussed in section 6.1.1, several errors during the experimental period influenced the water quality. Table 9 shows evidence when these
errors were influencing the water quality.

• 2/24/2017 – 3/10/2017: The system was still initializing the bio-layer of the sand filter, and biomaterial was found in the storage tanks
• 3/17/2017 – 3/24/2017: The system was operating, but not as efficiently as required as there was still bacteria present in the water
• 3/24/2017 – 4/14/2017: Errors in the system caused water quality to worsen. Leaves in gutters cause rotten water in the main storage tank. Incorrect sand sizes cause sand filters not to function as designed.
• 4/14/2017: The system was reset, and errors were addressed. The initialization period was allowed. No samples were taken
• 4/28/2017: Post-filtration showed zero bacteria count

Yeast and Fungi were never present in the water in any of the samples. The data set was used to determine when the SSF was functioning as per design.

The N/A notation indicates that no water sample test was taken at this location. This was due to the system being reset or still in the initialization phase. A sample test was not performed when there was no water in the storage tanks to test.

Figure 50 and Figure 51 indicate what the Micro count Combi slides looked like after they were inserted in two water samples and left to incubate for 48 hours.

7.3 Water Sample Analysis from Umgeni Water

After the Slow sand filter setup showed zero bacteria count, a water sample was taken to be analyzed according to the SANS 241 standards. A water sample was also taken from the forced filtration setup to be analyzed similarly. The results of this analysis will be used to compare the two filtration systems. The samples were sent off to Umgeni Water in Pinetown for analysis. Please find Umgeni Water’s certification of accreditation from SANS in Appendix K.

7.3.1 Results from Water Samples

Umgeni Water ran a test on three different water samples. A sample was taken from the pre-filtered water, the post-filtered water by SSF, and the post-filtered water from forced filtration. The results can be viewed below.

The results listed in Table 9 need to be compared to the SANS 241-2015 standards to ensure the water is safe for human consumption. The limitations of the SANS standards can be viewed in Table 10 below.

• Chronic Health: A chronic condition is a human health condition or disease that is persistent or otherwise long-lasting in its effects or a disease that comes with time. The term chronic is often applied when the course of the disease lasts for more than three months. Therefore, how the water
  will affect the long-term health of a user.
• Acute Health: Acute care is a branch of secondary health care where a patient receives active but short-term treatment for a severe injury or episode of illness, an urgent medical condition, or during recovery from surgery. In medical terms, care for acute health conditions is the opposite of chronic care or longer-term care. The effects the water will have on the short-term health of a user.
• Operational: This factor entails the water’s operational capability, i.e., whether it can be used in daily operations, such as cooking and cleaning.
• Aesthetic: The way the water looks to the human eye. Is it clear and clean or mercy and muddy?

A comparison is drawn between the SSF and forced filtration setups by comparing the effective removal capability of each system. The results from each system are then compared to the SANS limitations to ensure the water is of potable standards.

7.3.2 Discussion of Results

The results obtained in Tables 9 and 10 clearly show that the slow sand filter does not effectively filter water enough to produce potable water according to the SANS 241-2015 standards. It does not perform as well as the forced filtration in filtering rainwater.

These results prove that a slow sand filter cannot filter the rainwater in a rainwater harvesting system. If a slow sand filter is to be used, it must be done while incorporating filtering techniques used by the forced filtration setup. The forced filtration setup has proven to filter the water to the correct
standards.

8 Solar Powered Rainwater Harvesting System: Conclusion & Recommendations

The research and experimental study proved that a rainwater harvesting system can be designed to be solar-powered and provide enough water to satisfy the potable water needs of a small household. The study, however, did prove that using a slow sand filter in such a design is insufficient, as it does not meet the SANS 241-2015 standards.

The literature on the slow sand filter showed that it would be an ideally suited filtering technique for such a system due to its low cost, low maintenance and simplicity. Literature and research on slow sand filters were done in the early 19th century, and some of the research is outdated to a certain extent. Water quality tests have become more sophisticated and detailed than they were when slow sand filters were still being used on a large scale.

To operate as designed, the slow sand filter will have to work together with some aspects of the forced filtration technique. A layer of activated carbon can be inserted into the slow sand filter, and the solar pump can pump the water through a UV sterilization system. Making these corrections will improve the water quality but allow the system to use a small amount of energy and function for a small-scale design.

In 2017, we faced daily water shortage problems. The Western Cape region is suffering from severe droughts at the moment. Therefore, we need to start thinking smartly about water usage and supply. Rainwater harvesting is a viable solution to water needs in rural areas. Using old techniques with modern-day technology, the system design will make it an efficient and reliable source of potable water if the correct filtration is applied.

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