Each sewage treatment is designed to treat a particular effluent to a satisfactory standard so that the fluids can be discharged without causeing any harm to the enviroment and human kind. A higher standard of filtering may depend on the fragileness of the enviroment or the human population (for instance, certain enviroments may be suceptable to euthrophication by nitrates and phosphates leaching into a river or lake, creating an algae bloom and cultural eutrophication which only occurs due to mankind).
Index:
= Activated sludge sewage treatment
= Pre-treatment
= Primary sewage treatment
= Sludge disposal
= Secondary sewage treatment
= Tertiary treatment
= Sludge treatment
= Weather and flow functions
= Types of seage collection systemsActivated sludge sewage treatment:
This is a method of treating organic matter in sewage effluent, using digestion by aerobic bacteria. The effluent entering a sewage work does not just contain sewage. It will also contain fluids from a range of other uses such as toliet flushing water, water used for washing clothes, dishes and personal hydgene, road draingage water, ect. All these produce a large volume of effluent with a low consintration of sewage (typically 250 kg of water containing 2-3 kg of liquid and solid sewage per person per day).
Pre-treatment:
1. Screens and metal grills are used to supend floating debrey such as sticks, plastic, paper items, ect.
2. Grit traps collect stones and road grit (In most designs, channel widens so the flow velocity drops grit at bottem as there is not enough kinetic energy to carry it). It is later scraped out.
3. Materials are then collected and disposed at landfil site. May be insinerated to kill pathogens and reduce volume of solid waste.
4. Comminutors may be used to suspend faecal solids to increase the exposed surface area and therfore speed up later processes.
Primary treatment:
Primary sedimentation is the most improtant single process. The effluent is allowed to stand in large tanks so faecial solids can sink and be later removed in seperate treatment (Over 95% of the organic matter is removed from the effluent so the risk of deoxygenation in river or lake - which would suffocate fish and other organisms- is greatly reduced.
Sludge disposal:
1. Sludge collected from primary sedimentation tanks is mainly water, probably contains pathogens and its terrible smell makes storage/disposal in urban areas difficult.
2. Anerobic digestion reduces these problems. The sludge is digested by anerobic bacteria in a warm tank for about four weeks. During this time, most pathogens are killed and odours reduced. The remaining solids have settled and become more compact, meaning a smaller volume of waste for disposal.
3. Anerobic digestion produces sludge gas, which is mainly methane with some carbon dioxide. This can be used as a fuel to heat the digester, nearby buildings or to run converted diesel engines to produce electricity. This leaves the remaining sludge to be used in diffrent ways:
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Landfil: Simple and cheap. Wastes nutrients.
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Dumping at sea: Alright for costal communities. Banned in most contries to reduce pollution.
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Incineration: Needs fuel, hense expensive. Remaining ash can be dumped in landfil
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Agriculture: Sludge can be used as fertiliser. Has better C:N ratio and better odour than undigested sludge. Difficult however to apply a particular time to fields. Application during growing season would cover the crops, while application during winter may cause damage to damps soils as farm verchiles compact the soil.
Sludge may also contain heavy metals from urban wastes and road runoff. This could be used on crops and get into the food chain. This can largely be solved by adding lime; which reduces the solubility of heavy metals. Remaining pathogens can be killed by heat treatment.
Secondary treatment:
Remaining organic matter fluid effluent is broken down by bacteria. It is an aerobic process of which a shortage of oxygen can be a limiting factor. Large amounts of air may be mixed in by paddle wheels, air stones put in or pure oxygen pumped in to prevent this. Effluent contains suspended bacteria which must not be wasted. These settle out in secondary sedimentation tank, and are collected and returned to the airation tank as 'activated sludge'.
In most seage works the clear effluent from secondary treatment is discraged into rivers and lakes. It must have no pathogens or other organic material, but it may contain a small quantity of inorganic nutrients, which could cause cultural eutrophication if not regulated.
Trickling filter beds are an alternative to secondary treatment process that can be used instead of aeration tanks. Four rotating arms spray the liqued efluent over large cylindrical tanks containing lumps of a solid material such as gravel, coke or blast furnace clinker. This provides surfaces for bacteria, fungie, algae, worms and invetibrates to digest remaining orginaic matter. The thick bacterial mat on the inorganic medium can include anaerobic bacteria that denitrifys and reduces eutrophication caused by the effluent.
Tertiary treatment:
This is normally used if the effluent will be discharged into a particularly sensitive or important water body such as a very clean, clear river, which has important fisheries or wildlife. Phosphates can be removed by adding a solution of iron (III) sulfate. The phosphate group replaces the sulfate group, producing insoluble iron phosphate, which can be collected as a fine sediment. Any remaining suspended bacteria can be removed by passing the effluent through very fine sieves called micro-strainers or by adding a sterilising chemical such as chlorine.
Weather and flow functions:
If a sewage treatment plant recieves rainfall runoff, there may be a large increase of effluent volume following the storm. However, the amount of sewage itself will remain the same. Sewage plants are designed to cope with extra-flow, but not the maximum flow possible. This is costly and would only be benificial for rare and extreme storms. Instead, most have storm overflow tanks which can hold surplus effluent after a storm. This is then treated after effluent levels return to normal.
Types of sewage collection systems:Most types of sewage systems have been developed over the last 150 years. Underground pipes were laid to collect all the solid and liquid effluents. In the early days, they were dischared into rivers or seas. Perhaps onto fields and farmland. Eventually, sewage treatment works were added. This collection system produces a large quantity of dilute sewage effluent however.
A more effcient system is to separate waste collection systems.
System 1 - Collects small volumes of foul waste from toilets. This would require full treatment, but smaller volumes would mean easier to deal with as surface area is larger.
System 2 - Collects large volumes of clean water. This would require little treatment before discharged.
Edited by ex nihilo - 8/2/2012, 19:24
