Sewage treatment involves removal of contaminants and harmful elements from waste water, which is primarily discharged from households. Industrially viable waste water is produced by removal of contaminants through biological, physical and chemical processes.
Sewage, generally speaking, refers to waste water from household toilets, showers, baths, kitchens and sinks that drain into the sewers. Industrial liquid waste from power plants and refineries can also be termed as sewage. Industrial sewage differs from household sewage in its content. Industrial sewage often contains waste that is harmful to the environment if left untreated, whereas household sewage would cause general accumulation of waste water that leads to mosquitoes breeding, which in turn, can bring forth a plethora of diseases.
The most basic aim of any water treatment plant is to produce an affluent which causes as little harm as possible when released into the environment, hence reducing pollution caused by releasing untreated waste water into the surrounding environment.
Pretreatment is used to remove all substances from the sewage that may clog or damage the sewage carrying lines. This is done automatically by a mechanical screen bar in modern plants that serve big populations. The solid waste collected is either incinerated or disposed off in landfills.
Sewage treatment involves three main stages:
Primary treatment: This involves temporarily storing the sewage in a sedimentation basin where the solids can settle down to the bottom while fluids such as grease, oil and lighter solids float up to the surface. This separation allows floating and settled materials to be removed and the liquid can be either discharged or sent for secondary treatment.
Secondary treatment: This process removes suspended and dissolved organic matter. It is performed by indigenous micro-organisms in a controlled environment. Secondary treatment may require a step to separate the micro-organisms from the treated water before discharge or further treatment.
Tertiary treatment: This step is performed only when the water is to be discharged into a fragile or sensitive ecosystem like an estuary, low flow river or coral reef. Water is either disinfected physically or chemically using lagoons or microfiltration before discharging it into any water body like a river, stream or bay.
Odors emitted by a sewage plant are an indication of an anaerobic or septic environment. Early stages are more likely to produce foul smelling gases such as hydrogen sulphide. If the plant is near an urban area, then carbon reactors are used to treat the odours.
A conventional water treatment plant in Cork uses 30% of its annual operating cost on energy requirements. The requirements vary with type of treatment plant. For example, plants near wetlands have lower energy requirement than plants near urban areas because cost of transportation of water through pumps to water bodies is reduced. Similarly, treatment plants that produce biogas from anaerobic digestion can meet all their energy requirements in-house.