Valuation Office Agency
This section applies to all separately assessed sewage treatment works.
1.1 The function of a sewage treatment works
The primary function of a sewage treatment works (STW) is to treat effluent by the separation of the solids from the liquid and to make them safe before, in the context of liquid they are discharged, or in the context of sludge it is disposed of.
The area from which effluent is received by a particular STW is known as an agglomeration. Sewage treatment works are designed and operated to meet two specific characteristics. First, they are designed to deal with the specific nature of the effluent received from domestic waste water and industrial waste water within the agglomeration. Second, their design is dependent on the nature of the river, water course, estuary or sea into which the treated effluent is discharged.
The specific treatment facilities present on any STW will reflect these two characteristics. Environmental legislation has required effluent to be subject to an increasing level of treatment. The Sewage Undertakers have reacted by investing in plant to meet these demands. The main standards relate to the liquid discharged to the watercourse. These standards specify the discharge location, total volume of treated water permitted and the quality conditions of the discharged water, such as the biochemical oxygen demand (BOD), limits for suspended solids, ammonia, metals and toxic substances.
1.2 The Mechanics of Treatment
The level of treatment offered by a STW differs depending on the need of the receiving water. The range is from a simple septic tank which treats by settling, through to a sophisticated works dealing with a large urban population which will employ several stages.
1.2.1 Treatment of liquid effluent
The treatment of effluent, whether domestic or industrial is undertaken through a series of stages which typically involve slowing the passage of the effluent to allow for sedimentation or chemical precipitation and the addition or restriction of air to aid breakdown.
Treatment may be broken down into several stages; phrases such as preliminary, primary, secondary and tertiary are often used. As many works are very different in the type of treatment utilised these terms should be used with care as many variations exist.
The first stage orpreliminary treatmentoccurs at the start of the works. The effluent is delivered by a sewer to the area known as the Inlet Works. The inlet works will comprise a set of screens, located in a chamber. The function is to remove the larger solids from entering the works. The screens may be of different grades from parallel metal bars to a finer mesh either 6mm or 3mm depending on design. Debris collects on the screens and is removed often mechanically. The debris is then taken to landfill, off the site by skip. Located after the screens is the detritor. The screened effluent is then slowed down to allow grit and glass to deposit out, but not slow enough for other matter in suspension to be deposited. The grit and glass at the base of the detritor is removed by scrapers or suction. This debris is also taken by skip to landfill. Effluent is then taken to the next stage.
Primary treatmentremoves the solids in suspension by settlement. This stage takes place in a primary settlement tank. The effluent is held in a tank allowing the solids to precipitate to the bottom. As effluent is introduced to the tank, the cleaner liquid passes over the weir in the tank and on to the next stage. The solids that have fallen to the bottom of the tank are removed by scrapers that continually revolve around the tank. The solids are known as sludge and are taken to another part of the site for further treatment. Some STW do not have sludge treatment facilities - in these circumstances the sludge is conveyed by tanker to another works which does have these facilities.
Secondary treatmentis where effluent that has passed the earlier stages has air introduced to encourage the aerobic breakdown of harmful matter by aerobic action. This treatment may take place by several different methods.
Percolating filter beds: Here the effluent is distributed over the surface of a bed of graded inert medium (stone, slag, clinker) which usually has a depth of 1.8m. The distribution of the effluent is by moving arms that continuously sprinkle the effluent on the bed. The medium matter has a very high surface area which allows oxygen to be introduced to the effluent assisting the breakdown of harmful matter. A gelatinous film containing bacteria forms on the surface of the medium. The effluent from the base of the filter contains particles of detached film and other solids known as humus. This is removed at a further stage.
Aeration tank: These tanks deal with the same quality effluent that would pass through a percolating filter bed. The aeration tank contains only effluent through which air is introduced. Some methods employ air blown from the base of the tank, others use mechanical paddles to introduce oxygen. The effect is the same.
Tertiary treatmentor final polishing takes place with the effluent that has passed through the secondary treatment process. The purpose of tertiary treatment is mainly to reduce the Biochemical Oxygen Demand, and levels of suspended solids and other substances on the receiving water by the final effluent. Several different tertiary treatment processes are found on STW; irrigation, settlement, straining, filtration and disinfection, examples include:
Irrigation -Historically grass plots were employed but reed beds are now the modern equivalent. The flow enters the reed bed through an inlet that is designed to produce even distribution. The plants selected allow the flow through to encounter aerobic and anaerobic areas.
Settlement -Flocculent is added, causing the suspended solids to settle out. They are then removed. A flocculent is a substance that encourages the settling of suspended solids by attaching to them and depositing, the flocculent may be recycled.
Straining -The suspended solids are retained by the surface of the straining fabric. Eventually the accumulated solids block the mesh and must be removed. This is done by backwashing. Filtration Effluent is passed through a sand bed. The solids are retained within the bed and clear water passes through. Air is passed through the sand which separates the collected solids from the sand. The collected solids are then treated in the sludge line.
Disinfection -The Ultraviolet irradiation of treated sewage effluent renders the final effluent substantially disinfected. Required where discharges are made to bathing waters or shellfish growing areas.
1.2.2 Treatment of sludge
Sludge treatment occurs on a minority of STW sites in England and Wales. Most sites only separate the sludge from the liquid. Where a works does not have sludge treatment facilities, then the separated sludge will be taken by tanker or in some cases a dedicated pipe to a site that has the relevant facilities.
Various Statutory Instruments govern the way Sludge should be treated including: The Sludge (Use in Agriculture) Regulations 1989 as amended 1990. The Urban Waste Water Treatment (England and Wales) Regulations 1994.
The Urban Waste Water Treatment (England and Wales) Regulations 1994 prohibited the dumping of sludge at sea from the end of 1998, thus causing an increase in the need for sludge treatment facilities. There are several ways in which sludge can be treated. These include digestion, pasteurisation, composting, incineration and thermal drying, and these are considered below.
Sludge digestion
The most frequently encountered method of sludge treatment is digestion. The function of sludge digestion is three-fold, first to treat the harmful matter, second to reduce the mass of the solids, and third to facilitate the production of biogas. The harmful matter is subject to anaerobic degradation (without the presence of air). A typical Sludge digestion plant will comprise several stages: screening, blending, thickening, heating, primary digestion, secondary digestion, and finally dewatering.
Sludge will often be received to the site by two routes, the first indigenous sludge produced by the remainder of the works mainly through the primary settlement route. The second will be sludge brought to the site from other smaller works in the locality.
Sludge received from other sites will be subject to screening before being blended with the indigenous sludge and moved by pipe to the next stage thickening.
For sludge to be digested efficiently, the water content must be adjusted to around 94% - this is carried out in the thickening stage.
It is a requirement of the Department for Environment Food and Rural Affairs that sludge should be maintained during digestion at a specific temperature for a minimum specific period. Once the sludge has been subject to thickening it is then heated prior to being pumped into the primary digester. The heating takes place by pipes carrying hot water, the heat within the water is a waste product from the combustion of biogas produced by the primary digesters. The sludge is introduced into the primary digester in regular intervals in batchers.
Primary digesters can only operate full. As new material is introduced at the bottom of the digester the equivalent volume of primarily digested sludge is displaced at the top. Sludge within the primary digester undergoes anaerobic breakdown, and at various levels within the digester different microbes act to breakdown different material. The breakdown process releases bio gas which mainly comprises methane