Sewage treatment is the process of removing contaminants from waste water and household sewage, both runoffs (effluents), domestic, commercial and institutional. It includes physical, chemical and biological processes to remove physical chemical and biological contaminants. Its objective is to produce an environmentally safe fluid waste stream (or treated effluent) and a solid waste (or treated sludge) suitable for disposal or reuse (usually as farm fertilizer). Using advanced technology, it is now possible to re-use sewage effluent for drinking water, although Singapore is the only country to implement such technology on a production scale in its production of new water.

Sewage   is generated by residential institutional, commercial and industrial establishments. It includes household waste liquid from toilets, bath, showers, kitchens, sinks and so forth that is disposed of via sewers.
In many areas, sewage also includes liquid waste from industry and commerce. The separation and draining of household waste into grey water and black water is becoming more common in the developed world, with grey water being permitted to be used for watering plant or recycled for flushing toilet. Sewage may include storm water runoff. Sewage systems capable of handling storm water are known as combined sewer systems.
     As rainfall travels over roofs and the ground, it may pick up various contaminants including soil particles and other sediments, heavy metals, organic compounds, animal waste, and oil and grease.  Some jurisdictions require storm water to receive some level of treatment before being discharged directly into water ways. Examples of treatment processes used for storm water include retention basin, wetlands, buried vaults with various kinds of media filters, and vortex separator. Sewage can be treated close to where it is created, a decentralized system (in septic tanks, biofilter or aerobic treatment systems), or be collected and transported by a network of pipes and pump stations to a municipal treatment plant, a centralised system, sewage collection and treatment is typically subjected to local, state and federal regulations and standards. Industrial sources of sewage often require specialized treatment processes.
            Sewage treatment involves the removal of solids by physical screening or sedimentation. The removal of soluble and fine suspended organic pollutants by biological oxidation and adsorption processes. Both forms of treatment produce sludge as by–product and these have to be treated and used or disposed off in an economical and environmentally acceptable ways.
The following describes a typical sewage treatment sequence in practice, there are many process variations employed according to locality and the standard of effluent required. 

Screening: Large solids (plastic, rag, toilet paper residues) are removed first by mechanical screens. Traditionally, screening was used to remove only large material (> 25 – 30mm) in order to protect down stream operations. Nowadays, much finer screens (6mm. mesh) are commonly employed to remove small inert solids. The material retained (“screenings”) is usually washed to remove faecal matter and then compressed for disposal to land fill or to an incinerator.

            At the next preliminary stage, fine mineral matter (grit and sand), originating mainly from road run off, is allowed to deposit in long channels or circular traps. The retained solids are removed and usually sent to landfill for disposal.

Primary Sedimentation: The sewage passes into large sedimentation tanks to provide a quiescent settlement period of about 8 hours. Most of the solids settle to the bottom of the tanks and form a watery sludge, known as a “primary sludge”, which is removed for separate treatment. The sewage remaining after settlement has taken place is known as “settled sewage”.
            Secondary (Biological) Treatment; Settled sewage than flows to an aerobic biological treatment stage where it comes into contact with micro-organisms which remove and oxidize most of the remaining organic pollutants.
            At smaller works, the biological stage often takes the form of a packed bed of graded mineral media through which the sewage trickles and on the surfaces of which the micro-organism grow. At most larger works, the sewage is mixed for several hours with an aerated suspension of flies of micro-organisms (known as the activated sludge process). As well as removing most of the polluting organic matter, modern biological treatment can where necessary remove much of the nitrogen and phosphorous in the sewage, thus reducing the nutrient load on the receiving water.

Following secondary (biological) treatment, the flow passes to final settlement tanks where most of the biological solids are deposited as sludge (secondary sludge) while the clarified effluent passes to the outfall pipe for discharge to a water course. In the case of the activated sludge process, some of the secondary sludge is returned to the aeration tanks for further contact with the sewage. The secondary sludge from biological treatment also requires separate treatment and disposal and may be combined with the primary sludge for this purpose.

            In circumstances where the highest quality of effluent is required, a third (tertiary) stage of treatment can be used to remove most of the remaining suspended organic matter from the effluent before it is discharged to a water course. Tertiary treatment is effected by sand filters, mechanical filtration or by passing the effluent through a constructed wetland such as a reed bed or grass plot.

            All methods of sewage treatment generate organic sludge (or “biosolids”) as by-product and these must be managed separately from the liquid sewage. Raw (untreated) sludge has a very high oxygen demand and must not be allowed to enter the water environment. Sludge also contains pathogenic organisms.
There is, therefore, a need to deal with them in a way that permits their ultimate disposal in an environmentally acceptable and sustainable manner. The sludge “route” selected for a given sewage treatment works will depend on several factors including it’s location, the availability of suitable farm land, the characteristics of the sludge and the over all cost.
Sludge produced by sewage treatment are organic in nature and contain useful amount of plant nutrient such as nitrogen, phosphorous and essential trace elements. Therefore, the first objective should be to utilize the sludge as a fertilizer or soil conditioner on agricultural land. In their initial form, most raw (untreated) sludge have a high water content (96 - 99%), are putrescent and have an offensive odour.  They will also contain a variety of human and animal pathogen, derived from the contributing population. Various forms of treatment may be used to achieve volume reduction by removing some of the water content. Odour and pathogen reduction is achieved by stabilization and disinfection processes. In recent years, the control of odour emissions to the atmosphere has become an important requirement of sludge treatment.
The following outlines the more common type of treatment employed, of which various combinations are used according to the end product required.

            As a first stage of treatment, sludge is passed or subject to centrifugation to reduce it’s water content and volume by up to 50%. The separated liquor is returned to the sewage flow for treatment and the consolidated sludge passes forward for further processing.

In  Anaerobic digestion  (AD) consolidated liquid  sludge B retained in an airtight  tank  (digester)  and maintained   at 350c for 12-20 day . Under the anaerobic conditions in the tank various pathogen break  down about half of the sludge organic matter and convert it into a gas containing about 65% methane. The gas is used to heat the digester and, in some cases, also to fuel gas engines to generate electricity. The sludge resulting from anaerobic digestion is much less offense in odor than the untreated raw sludge and, with certain restriction, is generally suitable for use in agriculture in liquid or solid form. Further consolidation of sludge after digestion, to reduce its volume, is a common practice.

Either untreated or digested sludge may be converted from a liquid to a sludge “cake” by treating it first with conditioning chemical which releases much of the water initially bound to the organic matter. Much of the free water is then removed from the sludge in a filter press, a belt filter or a centrifuge. The resultant sludge cake will have only 20% of the volume and weight of the original sludge, thereby reducing subsequent handling and transport costs. The conversion of sludge to a solid form is essential prior to its disposal to landfill.

This involves the burning of the sludge at 850-900oc to destroy its organic content and to leave a smaller residue of mineral ash for final disposal, usually to landfill. Incineration is only suitable for large sewage works and is used when the option of agricultural use of the sludge is not practicable. The process is carried out under closely controlled conditions and is subject to strict environment regulation to ensure that the ambient air quality is not compromised by the combustion gases.      
Thermal Drying
Some sewage works in the UK employ thermal drying systems to convert the sludge to pelletised or granular form comprising about 90% solids. The heating involved also destroys pathogens. Thermally-dried sludges are used in agriculture or for amenity uses (for example, golf course, parks and other amenity areas)
Pasteurization (disinfection)
            To destroy all pathogens in liquid sludge, it is heated to about 700c for at least 30minutes after which it is cooled and subjected to anaerobic digestion. This combination of pasteurization and digestion produces an “enhanced treated” product (4) which enables it to be used more widely for various agricultural purposes.
Lime Stabilization
            At some smaller works, lime is added to liquid sludge to raise its pH to above 12.0 for several hours. The high alkalinity improves its odour and eliminates pathogens.
            Sieve: The influent sewage water passes through a bar screen to remove all large objects like cans, rags sticks, plastics packets etc. carried in the sewage stream. This is most commonly done with an automated mechanically raked bar screen in modern plants serving large populations, whilst in smaller or less modern plants; a manually cleaned screen may be used. The raking action of a mechanical bar screen is typically paced according to the accumulation on the bar screens and/or flow rate. The solids are collected and later disposed in a landfill or incinerated. Bar screen or mesh screens of varying sizes may be used to optimize solids removal. If gross solids are not removed, they become entrained in pipes and moving  parts of the treatment plant, and can cause substantial damage and inefficiency in the process.

Louis C. Umeh and Smart N. Uchegbu, (2011). Principles and procedures of Environmental impact assessment (EIA). Computer Edge publishers 26 Otasunmi St. Ejigbo Lagos. 2nd edition page 52-57.

Lansing M. Prescott, John P. Harley, Donal A. Klein, (1999). General Microbiology 4th edition page 868-873.  

Nduka Okafor, (1987). Industrial microbiology University of ife press Ltd, Ile - Ife, Nigeria 1st edition page 398-399.      

EPA, (2007). “Membrane Bioreactors” wastewater Management fact sheet.

Khopkar, S.M., (2004). Environmental pollution monitoring and control. New Delhii New age International. Page 299.

Smart N. Uchegbu, (2002). Environmental Management and protection spotlite publishers 11 Idodo street Achara layout Enugu 2nd edition page 177-191.
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