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Sewage treatment

From Academic Kids

Sewage treatment is the process that removes the majority of the contaminants from waste-water or sewage and produces both a liquid effluent suitable for disposal to the natural environment and a sludge. To be effective, sewage must be conveyed to a treatment plant by appropriate pipes and infrastructure and the process itself must be subject to regulation and controls. Other wastewaters require often different and sometimes specialised treatment methods.

Contents

Description

Sewage is the liquid waste from toilets, baths, showers, kitchens, etc. that is disposed of via sewers. In many areas sewage also includes some liquid waste from industry and commerce. In the UK, the waste from toilets is termed foul waste, the waste from items such as basins, baths, kitchens is termed sullage water, and the industrial and commercial waste is termed trade waste.

The division of household water drains into Greywater and Black water is becoming more common in the developed world, with greywater being permitted to be used for watering plants or recycled for flushing toilets. Much sewage also includes some surface water from roofs or hard-standing areas. Municipal wastewater therefore includes residential, commercial, and industrial liquid waste discharges, and may include stormwater runoff.

Sewerage systems that transport liquid waste discharges and stormwater together to a common treatment facility are called combined sewer systems. The construction of combined sewers is a less common practice in the U.S. and Canada than in the past and is no longer accepted within Building Regulations in the UK and other European countries. Instead, liquid waste and stormwater are collected and conveyed in separate sewer systems, referred to as sanitary sewers and storm sewers in the U.S. and as foul sewers and surface water sewers in the UK. Overflows from foul sewers designed to relieve pressure from heavy rainfall are termed storm sewers or combined sewer overflows.

As rainfall runs over the surface of roofs and the ground, it may pick up various contaminants including soil particles (sediment), heavy metals, organic compounds, animal waste, and oil and grease. Some jurisdictions require stormwater to receive some level of treatment before being discharged to the environment. Examples of treatment processes used for stormwater include sedimentation basins, wetlands, and vortex separators (to remove coarse solids).

The conventional sewage treatment process typically involves the following three stages:

  1. Primary Treatment - to settle out solids
  2. Secondary treatment - to remove the dissolved and emulsified components
  3. Tertiary treament - to make the effluent fit to be received in the environment.

Primary treatment

Primary treatment is to reduce oils, grease, fats, sand, grit, and coarse (settleable) solids.

Grit removal

This stage typically includes a grit channel where the velocity of the incoming wastewater is carefully controlled to allow grit and stones to settle but still maintain all organic material within the flow. Grit and stones need to be removed early in the process to avoid damage to pumps and other equipment in the remaining treatment stages.

Screening or maceration

The grit free liquid is then passed through fixed or rotating screens to remove floating and larger material such as rags. Screenings are collected and may be returned to the sludge treatment plant or may be disposed of off site by landfilling or incineration. Maceration, in which solids are cut into small particles through the use of rotating knife edges mounted on a revolving cylinder, is used in plants that are able to process this particulate waste. Macerators are, however, more expensive to maintain and are less reliable than physical screens.

Sedimentation

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Primary sedimentation tank at a rural treatment plant

In almost all plants there is a sedimentation stage where the sewage is allowed to stand in large tanks so that faecal solids can settle and floating material such as grease and plastics can rise to the surface and be skimmed off. The main purpose of the primary stage is to produce a generally homogeneous liquid capable of being treated biologically together with a sludge that can be separately treated or processed. Primary settlement tanks are usually equipped with mechanically driven scrapers that continually drive the collected sludge towards a hopper in the base of the tank from where it can be pumped to further sludge treatment stages.

Secondary treatment

Secondary treatment is designed to substantially degrade the biological content of the sewage. The majority of municipal and industrial plants treat the settled sewage liquor using aerobic biological processes. For this to be effective, the biota require both oxygen and a substrate on which to live. There are number of ways in which this is done. In all these methods, the bacteria and protozoa consume biodegradable soluble organic contaminants (e.g. sugars, fats, organic short-chain carbon molecules, etc.) and bind much of the less soluble fractions into floc particles.

Roughing Filters

Roughing filters are intended to treat particularly strong or variable organic loads. They are typically tall, columnar filters filled with open synthetic filter media to which sewage is applied at a relatively high rate. The design of the filters allows high hydraulic loading and a high flow-through of air. The resultant liquor is usually within the normal range for conventional treatment processes.

Activated sludge

Activated sludge plants use a variety of mechanisms and processes to use dissolved oxygen to generate a biological floc that substantially removes organic material. It also traps particulate material and can, under ideal conditions, convert ammonia to nitrite and nitrate and ultimately to nitrogen gas, see also denitrification.

Filter Beds

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Trickling filter bed using plastic media

In older plants and plants receiving more variable loads, trickling filter beds are used where the settled sewage liquor is spread onto the surface of a deep bed made up of coke (carbonised coal), rocks or specially fabricated plastic media with high surface areas. The liquor is distributed through perforated rotating arms radiating from a central pivot. The distributed liquor trickles through this bed and is collected in drains at the base. These drains also provide a source of air which percolates up through the bed, keeping it aerobic. Biological film comprising of bacteria, protozoa and fungi forms on all the available surfaces and this provides the required biological treatment capability to effect the reduction in organic content.

Rotating Plates and Spirals

In some smaller plants slowly revolving plates or spirals are used which are partially submerged in the liquor. A biotic floc is created which provides the required substrate.

Secondary sedimentation

The final step in the secondary treatment stage is to settle out the biological floc or filter material and produce an effluent with very low levels of organic material and suspended matter.

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Secondary Sedimentation tank at a rural treatment plant

Tertiary treatment

Tertiary treatment provides a final stage to raise the effluent quality to the standard required before it is discharged to the receiving environment (sea, river, lake, ground, etc.) More than one tertiary treatment process may be used at any treament plant. If disinfection is practiced, it is always the final process.

Effluent polishing

Filtration

Slow sand filtration removes much of the residual suspended matter. Filtration over activated carbon removes residual toxins.

Lagooning

Lagoons provides settlement and further biological improvement through storage in large man-made ponds or lagoons.

Constructed wetlands

These include engineered reed beds and a range of similar methodologies, all of which provide a high degree of aerobic biological improvement and can often be used instead of secondary treatment for small communities, also see phytoremediation.

Nutrient removal

Wastewater may also contain high levels of nutrients (nitrogen and phosphorus) that in certain forms may be toxic to fish and invertebrates at very low concentrations(e.g. ammonia) or that can create nuisance conditions in the receiving environment (e.g. weed or algal growth). Although the growth of weeds and algae may seem to be primarily an aesthetic issue, algae can produce toxins, and in dying their decay and consumption by bacteria in the environment can result in the depletion of oxygen in the water and the possible consequential suffocation of fish. Where receiving rivers discharge to lakes or shallow seas, the added nutrients can cause severe and sometimes irreversible eutrophication with the loss of many sensitive clean water species. The removal of nitrogen and/or phosphorus from wastewater can be achieved either biologically or by chemical precipitation treatment processes.

Nitrogen removal

Biological treatment of nitrogen generally involves creating conditions within the treatment process for bacteria to convert the ammonia to nitrate, and then allowing other bacteria to reducing the nitrate to nitrogen gas, which is released to the atmosphere. Sand filters, lagooning and the use of reed beds can all be used to reduce nitrogen. Sometimes the conversion of toxic ammonia to nitrate alone is referred to as tertiary treatment.

Phosphorus removal

The biological treatment of wastewater to remove phosphorus also involves the design and creation of specific environmental conditions within a treatment plant to enable specific bacteria to bio-accumulate large quantities of phosphorus. When the bacteria containing the phosphorus are removed, the resulting bacterial biosolids often have a high fertilizer value. Phosphorus can also be removed by chemical precipitation using (commonly) salts of iron (i.e. ferric chloride) or aluminum (i.e. alum). The resulting chemical sludge, however, is difficult to dispose of, and the use of chemicals in the treatment process is expensive and makes operation difficult and often messy.

Disinfection

Disinfection substantially reduces the numbers of living organisms in the water to be discharged. The effectiveness of disinfection depends on the quality of the water being treated, the type of disinfection being used, the application rate, the contact time and environmental variables. Turbid water will be treated less successfully since solid matter can shield organisms, especially from Ultraviolet light or if contact times are low. Generally, short contact times, low doses and high flows all mitigate against effective disinfection. Common methods of disinfection include ozone, chlorine, or UV light. Chloramine, which is used for drinking water, is not used in waste water treatment because of its persistence.

Chlorination remains the most common form of wastewater disinfection in North America due to its low cost and long-term history of effectiveness. One disadvantage is that chlorination of residual organic material can generate chlorinated-organic compounds that may be carcinogenic or harmful to the environment. Residual chlorine or chloramines may also be capable of chlorinating organic material in the natural aquatic environment. Further, because residual chlorine is toxic to aquatic species, the treated effluent must also be chemically dechlorinated, adding to the complexity and cost of treatment.
Ultraviolet Light is becoming the most common means of disinfection in the UK because of the concerns about the impacts of chlorine in chlorinating residual organics in the wastewater and in chlorinating organics in the receiving water. Ultraviolet radiation is used to damage the genetic structure of bacteria, viruses, and parasites, making them incapable of reproducing. The key disadvantages of ultraviolet disinfection is the need for frequent lamp maintenance and replacement and the need for a highly treated effluent to ensure that the ultraviolet radiation can get through to any microorganisms present (i.e. solids present in the treated effluent may protect microorganisms from the U.V. radiation).
Ozone O3 is generated by passing oxygen O2 through a high voltage potential resulting in a third oxygen atom becoming attached and forming O3. Ozone is very unstable and reactive and oxidizes most organic material it comes in contact with, destroying many disease-causing microorganisms. It has the added bonus of removing other wastewater components such as colour. Ozone is considered to be safer than chlorine because, unlike chlorine which has to be stored on site, ozone is generated as it is required.

Package plants and batch reactors

In order to use less space, treat difficult waste, deal with intermittent flow or achieve higher environmental standards, a number of designs of hybrid treatment plants have been produced. Such plants often combine all or at least two stages of the three main treatment stages into one combined stage. Typically, activated sludge is mixed with raw incoming sewage and mixed and aerated. The resultant mixture is then allowed to settle producing a high quality effluent. The settled sludge is run off and re-aerated before a proportion is returned to the head of the works. This is the principle behind modern Sequental Batch Reactors (SBRs) and a range of specialist applications. The disadvantage of such processes is that a high level of control of timing, mixing and aertaion is required which can only be achieved by computer control linked to a range of sensors in the plant. These plants are technologically sophisticated and are unsuited to environments where such control may be unreliable or where the power supply may be intermittent. SBR plants are now being deployed in many parts of the world including North Liberty, Iowa, and Llanasa, North Wales.

Sludge treatment

The coarse primary solids and secondary biosolids (bacteria) accumulated in a wastewater treatment process must be treated and disposed of in a safe and effective manner. This material is often inadvertently contaminated with toxic organic and inorganic compounds (e.g. heavy metals). The purpose of digestion is to reduce the amount of organic matter and the number of disease-causing microorganisms present in the solids. The most common treatment options include anaerobic digestion, aerobic digestion, and composting.

Anaerobic digestion

Anaerobic digestion is a bacterial process that is carried out in the absence of oxygen. The process can either be thermophilic digestion (in which sludge is fermented in tanks heated to about 38°C) or mesophilic digestion (cold digestion of sludge where sludge is maintained in large tanks for weeks to allow natural mineralisation of the sludge). Thermophilic digestion generates biogas with a high proportion of methane that may be used to both heat the tank and run engines or microturbines for other on-site processes. In large treatment plants sufficient energy can be generated in this way to produce more electricity than the machines require. The methane generation is a key advantage of the anaerobic process. Its key disadvantage is the long time required for the process (up to 30 days) and the high capital cost.

Aerobic digestion

Aerobic digestion is a bacterial process that runs in the presence of oxygen. Under aerobic conditions, bacteria rapidly consume organic matter and convert it into carbon dioxide. Because the aerobic digestion occurs much faster than anaerobic digestion, the capital costs of aerobic digestion are lower. However, the operating costs are characteristically much greater for aerobic digestion because of the need to add oxygen to the process.

Composting

Composting is also an aerobic process that involves mixing the wastewater solids with sources of carbon such as sawdust or wood chips. In the presence of oxygen, bacteria digest both the wastewater solids and the added carbon source and, in doing so, produce a large amount of heat. Properly designed and controlled, the heat generated can be sufficient to significantly destroy a sufficient number of the disease-causing microorganisms to enable the resulting composted product to be safely used as a soil amendment material (with similar benefits to peat) for agricultural use.

Both anaerobic and aerobic digestion processes can result in the destruction of disease-causing microorganisms and parasites to a sufficient level to allow the resulting digested solids to be safely applied to land or used for agriculture as a fertilizer provided that levels of toxic constituents are sufficiently low.

The choice of a wastewater solid treatment method depends on the amount of solids generated and other site-specific conditions. However, in general, composting is most often applied to smaller-scale applications followed by aerobic digestion and then lastly anaerobic digestion for the larger-scale municipal applications.

Thermal depolymerization

Thermal depolymerization uses hydrous pyrolysis to convert reduced complex organics to oil. Basically, the premacerated, grit-reduced sludge is heated to 250C and compressed to 40 MPa. The hydrogen in the water inserts itself between chemical bonds in natural polymers such as fats, proteins and cellulose. The oxygen of the water combines with carbon, hydrogen and metals.

The result is oil, light combustible gases such as methane, propane and butane, water with soluble salts, carbon dioxide, and a small residue of inert insoluble material that resembles powdered rock and char.

All organisms and organic toxins are destroyed. Inorganic salts such as nitrates and phosphates remain in the water after treatment at sufficiently high levels that further treatment is required.

The energy from decompressing the material is recovered, and the process heat and pressure is usually powered from the light combustible gases. The oil is usually treated further to make a defined useful light grade of oil, such as no. 2 diesel and no. 4 heating oil, and then sold.

The process can be made quite efficient.

Sludge disposal

When a liquid sludge is produced, further treatment may be required to make it suitable for final disposal. This includes lagooning in drying beds to produce a cake that can be applied to land or incinerated; pressing, where sludge is mechanically filtered, often through cloth screens to produce a firm cake; or liquid injection to land or liquid disposal to landfill. There are concerns about sludge Incineration because of the contents of the emmissions to air, along with the high cost of supplemental fuel, making this a less attractive and less commonly constructed means of sludge treatment and disposal.

Treatment in the receiving environment

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The outlet of a waszewater treating plant flows into a small river

Many processes in a wastewater treatment plant are designed to mimic the natural treatment processes that occur in the environment, whether that environment is a natural water body or the ground. If not overloaded, bacteria in the environment will consume organic contaminants, although this will reduce the levels of oxygen in the water and may significantly change the overall ecology of the receiving water. Native bacteria feed on the organic contaminants, and disease-causing microorganisms are reduced by natural environmental conditions that are hostile to these organisms (microbial predation, ultraviolet radiation, etc.) Consequently in cases where the receiving environment provides a high level of dilution, a high degree of wastewater treatment is not necessarily required. However, recent evidence has demonstrated that very low levels of certain contaminants in waste water, including hormones (from animal husbandry and residue from human birth control pills) and synthetic materials such as pthalates that mimic hormones in their action, can have an unpredictable adverse impact on the natural biota and potentially on humans if the water is re-used for drinking water.

See also

External links


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