This article aims to present a general overview of a packaged sewage treatment process and sewage transportation network. The network described here is mini to medium size, usually seen in Field Logistic Base (FLB), Medium size residential estates etc.
The sewage transported mainly comprises excretes from toilets, water from bathrooms, kitchen and laundry rooms. Note that the sewage network may also transport some element of run-off water from facilities; however, this is assumed not to contain pollutant (such as hydrocarbons) that requires special treatment.
The sewage/effluent network description will cover lift stations, piping materials utilised for sewage network etc.
The treatment package described in this article is a packaged unit, and the technology employed may vary across different Vendors.
Sewage is waste matter from domestic or industries that are carried away in sewers for dumping or conversion into a form that is not toxic to the environment.
Effluent is treated or untreated wastewater that flows from a process to the surrounding environment.
Sewage/wastewater is generated in all facilities that have toilets, kitchens, laundry rooms etc. The quantity of sewage generated is a function of the capacity of the facility. Sewage is a combination of liquid, solid and suspension that requires treatment.
The liquid is separated from the solid, and each component is treated. The treated solid, including dried sludge, can be used for landfills or usually dumped at a designated location. In contrast, the treated liquid (effluent) is transported to flowing streams or recycled for further use.
Field Logistic base, estates, factories etc., usually have a central sewage treatment facility. Waste from all buildings is channelled through piping into chambers and further into lift stations. The waste is transported into a pump station from the lift stations, which further transport the sewage into a central treatment facility.
3 SEWAGE treatment facility and NETWORK OVERVIEW
A sewage treatment facility consists of a piping system connecting toilets, kitchens, laundry rooms, inspection chambers, lift stations. Pump stations and sewage treatment package.
The subsections below briefly describes the components of the sewage network.
Figure 1: Typical Sewage/Effluent Network
3.1 Piping Network
Piping are the arteries connecting toilets, kitchen sinks, wash hand basins, floor drains, inspection chambers, lift stations, pump stations and the sewage treatment package.
The piping connected to the toilets are mainly made of PVC (polyvinyl chloride) materials. The piping from the toilets are connected to inspection chambers, solid materials trapped in the chamber are removed periodically. Piping from the toilet to the chamber should slope toward the chamber, and the piping out of the chamber should slope outward the chamber. The piping entering the chamber should have a higher elevation than the one exiting the chamber.
3.1.1 Piping Materials Requirement
Various piping materials are used for the sewage piping network; however, some materials such as carbon steel are rarely used due to the fluid’s high corrosive nature. Non-metallic piping (PVC pipes) are mainly used for all plumbing and piping works within the building areas. Central piping within the networks connecting the lift stations and treatment skid are mostly made of non-metallic piping such as HDPE or GRP pipes. Stainless can be used mainly on pump discharge piping; however, they are not as common as non-metallic piping due to the high cost. Lined carbon steel piping may also be used to transport sewage piping if the integrity of the lining is guaranteed.
3.2 Inspection Chambers
Inspection chambers are access points to the sewage piping network. Piping from toilets, floor drains, laundry room may be connected to a chamber depending on the flow. The chamber should be sized for maximum flow from all the sources of flow into the chamber. Flow into the inspection chamber is usually by gravity; therefore, the piping should slope toward the chamber. A single pipe transfers the sewage from a chamber to another larger chamber. The larger inspection chambers are those connected to the lift stations. The number of inspection chambers is dependent on the size of the sewage network. All inspection chambers should have a removable cover for easy access to the chamber.
3.3 Lift Stations
The lift station serves as a pressure or flow booster station. Sewage piping networks are designed for gravity flow. Lift stations are necessary to raise the piping elevation after it has reached a defined depth. The number of lift stations is a function of the length of the piping network.
A lift station is a deep chamber of appropriate dimensions. A pump is installed in the chamber to boost the flow by raising the head of the sewage.
Why are lift stations necessary, and how do they work: Usually, sewage piping are buried; therefore, the minimum burial depth at any location, depending on local regulation, might be approximately 0.8m. The slope of the piping system might range from 1: 25 to 1:50, meaning for every 50m length of piping, the depth of the buried pipe increases by 1m. Assuming the total length of the sewage piping between a lift station and a sewage treatment unit is 1000m, this implies the minimum piping depth based on the slope of 1: 50 is 20.8m. This depth is unachievable; therefore, intermediate lift stations are required at an interval to limit the maximum piping depth to ~3m. At each lift station, the pumps are installed such that the discharge piping elevation is raised to 0.8m, as previously explained.
Figure 2: Elevation View of a Lift Station
3.3.1 Lift Station Pumps
The pumps are the only equipment installed in the lift station. The philosophy may be one running on full capacity while the others are on standby. There must be at least a spare pump in every lift station. Depending on the flow requirement, more than two pumps may be required. The pumps are mainly vertical submersible pumps which imply the entire pumps and their drivers; usually, electric motors are submerged. Another pump design configuration is having the pump submerged in the fluid while the driver (Electric motor) is connected to the pump via a shaft; this implies the electric motor is not submerged. The discharge piping is connected to the outlet nozzle of the pump.
Level switches usually control the pumps, i.e. they start running automatically when the sewage volume reaches a set high level set point and stop when the level reaches the low-level set point.
Figure 3: Typical Lift Station Pump
3.4 Inlet Bar Screen
Inlet bar screen may be installed at strategic locations, i.e. upstream of the lift stations or upstream the waste treatment skid. The sewage flows through the inlet bar screen where floating materials, such as clothes, papers, wood, and kitchen, refuse, and other solid materials are trapped and removed from the sewage stream.
The bar screen usually consists of ½ inch diameter bars spaced 1 inch apart that trap all the solid materials; however, the design may vary.
Figure 4: Typical Inlet Bar Screen
3.5 Main Pump Station
The pump station is usually installed upstream of the Sewage treatment unit because the inlet nozzle to the skid is at the upper part of the unit, while the sewage piping connection to the nozzle is below ground. The pumps are required to raise the liquid head for easy discharge into the treatment skid. Just like the lift station, there must be spare pumps/ pumps installed in the pump station. The pump station is like the lift stations; the only difference is the pump discharge piping is connected to the sewage treatment skid, while the pumps may be horizontal or vertical depending on the design.
3.6 Sewage Treatment Unit
Depending on the design philosophy, the treatment unit may utilise different technology. Some treatment unit utilises chemical for treatment other utilises naturally occurring biological process. The biological process is the most common technique employed because it has been proven efficient and economically viable.
There are two types of biological processes. They entail the use of anaerobic or aerobic bacteria to break down the waste pollutants. In any of the two processes, the microorganisms (bacteria) utilise the organic pollutants in the wastewater as their energy source of energy for their cell life activities.
Anaerobic waste sewage treatment occurs when there is a lack of oxygen present in the wastewater. Anaerobic treatment occurs under septic conditions is not widely used to treat sewage because the process converts hydrogen and sulphur into hydrogen sulfide and methane. Carbon may also be converted into organic acids that make the water more difficult to treat, promoting odour formation. The anaerobic process requires a longer retention time for the bacteria to break down the pollutants, requires larger tankage than aerobic treatment systems. There is an incomplete breakdown of the pollutant.
The aerobic waste treatment process occurs in the presence of oxygen. The Aerobic bacteria introduced into the sewage treatment unit (Aeration Chamber) rapidly consume the organic waste pollutants in the sewage and release carbon dioxide and water vapour as by-products of the synthesis process. The aerobic process does not result in an explosive atmosphere because methane is not generated.
Also, because of the complete breakdown of the waste pollutants and the release of only carbon dioxide and water, the odour is not generated from the process. Note that aerobic treatment will convert into an anaerobic process if there is no oxygen supply for about twelve to thirty-six hours.
The treatment unit described in this article is a containerised unit that is quickly delivered to the site and relocated. The unit described in this article utilises aerobic treatment process with sub-surface aeration.
The conventional treatment unit used for large sewage systems is not containerised. They are made of concrete.
Figure 5: Typical Sewage Treatment Process
Figure 6: Containerised Sewage Treatment Package
Below are the different processing units of a containerised sewage treatment facility
3.6.1 Bar Screen
Refer to session 3.4. for details of the inlet bar screen. Sewage treatment packages usually have an inbuilt bar screen primarily to remove coarse materials, including rags, plastic bags, etc., from entering the sewage treatment unit.
3.6.2 Equalisation Chamber
After the sewage/wastewater has been screened, the wastewater is transferred to the Equalization Chamber, which normalises the flow by eliminating surges that may occur during peak flow times. A set of Equalisation Pumps is installed within the chamber to achieve flow control in this chamber. The pumps transfer plant influent through a flow control box. The box is composed of an adjustable overflow broad weir and a V-notch discharge weir. The broad weir is adjusted to ensure that only a measured amount of pumped influent/wastewater/sewage is discharged through the V-notch weir to the aeration chamber. The remaining influent is recycled back into the equalisation chamber.
3.6.3 Aeration Chamber
Aeration is the process of introducing air into wastewater treatment systems to aid microbial growth to allow aerobic biodegradation of the pollutant components of the sewage. The air is usually supplied from an external source (Air blowers) continuously to boost the degradation of the waste.
The size of the aeration chamber is based on the daily average flow volume of the sewage generated that will be processed. Depending on the type of treatment system, the influent might be retained in the aeration chamber for 18 to 24 hours. This period is required to have an extended aeration biological treatment process that allows for the complete conversion of organic pollutants to non-pollutant. The extended biological treatment process does not generate excess sludge. The sewage entering into the chamber is mixed with water that contains a large concentration of very active aerobic bacteria that consume the organic waste material in the sewage.
3.6.4 Air Supply System
The air supply system is primarily made up of air blowers and the air diffuser. Diffusers are installed in the aeration chamber, while the air blowers are installed in an enclosure within the package. Sub-surface aeration is used in most sewage treatment units meaning the air is introduced into the aeration chamber from below the water.
The Air Blowers compress the air delivered through airlines to the diffusers installed in a pattern on the bottom of the aeration chamber. The diffusers have tiny holes that generate bubbles that contact the water, thereby aerating the water. The air from the diffusers also keeps the bacteria in suspension, provides the agitation necessary to prevent the solids from settling at the bottom of the aeration chamber. There are two types of diffusers: fine bubble and course bubble.
Fine bubbles diffusers have many tiny holes and generate small bubbles compared to coarse bubble diffusers. The tiny bubbles provide maximum contact surface between the oxygen and water. The air from fine bubble diffusers also takes a longer time to get to the surface of the aeration chamber. Fine bubbles diffusers provide better utilisation of oxygen and, therefore, more efficient in oxygen usage.
Coarse Bubble Diffusers have fewer holes and generate larger bubbles. The bubbles travel faster to the surface than the tiny bubbles generated in fine bubbles diffusers. The large bubbles generate turbulence and better mixing of the air and wastewater. In a situation where mixing the air and the wastewater is more important coarse bubble diffusers are a better option.
3.6.5 Clarification Chamber
The organic pollutants are separated from the water in the clarification chamber.
As new sewage is pumped into the aeration chamber, aerated and broken down sewage is pushed into the clarification chamber. The clarification chamber may be designed to hold the fluid for about 6 hours to keep the fluid as still as possible despite the inflow from the aeration chamber and outflow from the clarification chamber.
The clarification is designed such that the top of the clarification chamber is below the waterline of the aeration chamber. This is to ensure that the clarification chamber is kept under head pressure.
The clarification chamber is divided into two sections by a baffle (clarifier baffle). The inside baffle area is towards the aeration chamber, and the outside baffle area is towards the fluid disinfection chamber. The baffle prevents the fluid from leaving the clarification chamber without the proper retention time been attained, thereby promoting the proper settling of flocs/sludge/solids to the bottom. As the organic pollutants/sludge settles to the bottom of the clarifier, a zone of clear liquid is formed on the outside baffle. New fluid flowing into the clarification chamber gradually push the clear liquid into the Media Chamber
Most of the sludge in the clarification chamber is continuously drawn back into the aeration chamber by an airlift sludge return line. Also, floating materials in the inside baffle are drawn back into the aeration by a skimmer return line. The sludge and the floating materials are then subjected to the same synthesis process as the bacteria’s.
Note: The air blowers supply the required air to the sludge return line and the skimmer line.
3.6.6 Media Chamber
Transparent liquid from the clarification chambers flows into the media chamber, where the fluid is polished. The fluid is made to pass through a media bed, thereby allowing for the settling and removing of any solids that could pass through the clarification chamber. Media Chamber is optional.
3.6.7 Disinfection Chamber
The last treatment activity occurs in the disinfection chamber. Liquid from the media chamber flows into the disinfection chamber, where an appropriate amount of chemical is injected. Liquid chlorine is used for disinfection. The dosing pump is set to deliver an appropriate volume of liquid chlorine into the disinfection chamber.
Must provide adequate time for the chlorine to come in direct contact with all the microorganisms in the liquid; else disinfection will be incomplete. After the chemical dosing, the effluent is disposed of or reused. A test should be carried out periodically on the effluent to confirm the composition of the effluent been discharged into the environment.
3.6.8 Sludge Holding/Processing Unit
As previously stated, most of the sludge is lifted back into the aeration chamber for reprocessing; however, excess sludge should be discharged from the system into the sludge processing unit. The excess sludge is sent into the sludge processing unit, where most of the liquid is allowed to settle. The liquid is sent back into the aeration chamber while the sludge is sent into a sludge drying bed/unit. The dried sludge may be utilised for landfill or disposed of appropriately.
3.6.9 Control Panel
The packaged unit usually have control panels (Electrical and instrumentation) where the equipment is controlled. The panels are installed within an enclosure or outside close to the treatment unit.