Sewage treatment

. Sewage treatment is the process that removes the majority of the contaminants from sewage and produces an liquid effluent suitable for disposal to the natural environment and also produces a sludge.

Historical sewage treatment

As recently as 100 years ago in major cities of developed countries, and up to the present day in many parts of the world, the primary concern with sewage was the matter of conveying it away from inhabited areas. Aside from its unpleasant odor, even early humans were aware that health problems arose when human waste was left to contaminate drinking water supplies.

Therefore, the historical focus of sewage treatment was on conveyance of raw sewage to a natural body of water, such as a river or ocean, where it would be satisfactorily diluted and dissipated. Early human habitations were often built next to water sources. Rivers could double as a crude form of natural sewage disposal and treatment.

Higher population densities required more complex sewer collection and conveyance systems in order to maintain (somewhat) sanitary conditions in crowded cities. The city of Mohenjo-daro, constructed around 2600 BC had brick lined sewage drains and also had outdoor toilets connected to this network. Ancient Minoan civilization had stone sewers that were periodically flushed with clean water.

Roman towns and garrisons in the UK between 46 BC and 400 AD had complex sewer networks sometimes constructed out of hollowed out Elm logs which were shaped so that the butted together with the down-stream pipe providing a socket for the upstream pipe.

A significant development was the construction of a network of sewers to collect waste water. In some cities, including Rome and Istanbul (Constantinople), networked ancient sewer systems continue to function today as collection systems for those cities' modernized sewer systems. Instead of flowing to a river or the sea, the pipes have been re-routed to modern sewer treatment facilities.

However, many cities had no sewers and relied on nearby rivers or occasional rain to wash away sewage. In some cities, waste water simply ran down the streets, which had stepping stones to keep pedestrians out of the muck, and eventually drained as runoff into the local watershed. This was enough in early cities with few occupants but the growth of cities quickly overpolluted streets and became a constant source of disease. Even as recently as the late 19th Century sewerage systems in parts of the highly industrialised UK were so inadequate that water borne diseases such as Cholera and Typhoid were still common. In Merthyr Tydfil, a large town in South Wales, most houses discharged their sewage to individual cess-pits which persistently overflowed causing the pavement (sidewalk) to be awash with foul sewage

Wastewater (sewage)

Wastewater 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 basins, baths, kitchens etc is termed sullage water and the industrial and commercial waste is termed trade waste. 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. 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), metals, organic compounds, animal waste, and oil and grease. Some jurisdictions, such as certain communities located in southern California, 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).

Wastewater conveyancing

Wastewater collection systems consist of buried pipelines which may convey the wastewater by gravity, or the wastewater may be pumped (i.e. forcemains), or for some low-lying communities it may be conveyed by vacuum. Where pipeline excavation is difficult (e.g. rock) or there is limited topographic relief (i.e. flat terrain), gravity collection systems may not be practical and the sewage must be pumped through the pipeline to the treatment plant. These pipelines may range in size from pipes of six inches (150 mm)in diameter to concrete lined tunnels of up to thirty feet (10 Metre) in diameter.

Wastewater contaminant source control

Wastewater is collected by sanitary sewer systems (foul sewerage system in the UK) and is typically conveyed to a centralized wastewater treatment facility where it is treated in several stages to reduce the level of some contaminants. Because industrial liquid waste may contain a wide range of chemicals, solvents, and other contaminants that cannot be effectively removed by the centralized wastewater treatment plant, industries are often required to pre-treat their liquid wastes prior to discharging to sewer. Most major municipal jurisdictions in North America with significant industrial liquid waste sources have discharge bylaws that restrict the quantity and maximum level of specific contaminants that may be discharged to sewers. In the UK charges are levied on industries who discharge industrial waste to the sewers. These charges relate to volume, organic strength and their toxic metal content.

Conventional wastewater treatment

Municipal wastewater treatment facilities can only remove a limited range of contaminants from wastewater. The treatment process typically involves the following three stages:

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

• This stage typically includes a grit channel to remove grit and stones that might damage pumps and equipment before it is passed on through the remaining treatment stages. This is followed by screening or maceration equipment to remove light solids such as rags or reduce them to small particles capable of further treatment. In almost all plants this is followed by a sedimentation stage where the sewage is allowed to stand in large tanks so that faecal solids can settle and floating material such a grease and plastics can rise to the surface and can 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.

2. 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
  • Activated sludge - The settled sewage liquor is run into deep tanks and air or oxygen is forced through the liquor from diffusion blocks on the base of the aeration tanks or alternatively by the use of deep aeration cones which draw up the liquor and spin it into the air. In both cases a biotic floc is created which provides the required substrate. In some areas where more land is available sewage is treated in large round or oval ditches with one or two rotating paddles which drive the liquor around the ditch and provide aeration. These are Pasveer ditches and have the advantage that they are relatively easy to maintain and are resilient to shock loads that often occur in smaller communities (i.e at breakfast time and in the evening)
  • In older plants and plants receiving more variable loads, filter beds are used where the settled sewage liquor is spread onto the surface of a deep bed made up of coke (carbonised coal) or rocks with high surface areas. The liquor trickles through this bed of rocks 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 rock surfaces and this provides the required biological treatment capability to effect the reduction in organic content
  • There are a wide range of other types of plants, often serving small communities or industrial plants. In some smaller plants slowly revolving plates or spirals are used which are partially submerged in the liquor. Others use hybrid treatment processes often involving the use of aerobic sludge to treat the incoming sewage. In such plants the primary stage may be omitted.

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. 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. In activated sludge plants, part of the settled material, the sludge, is returned to the head of the aeration system to re-seed the new sewage entering the tank.

3. '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.)

• Effluent polishing
• *Sand filtration removes much of the residual suspended matter
  • Lagooning provides settlement and further biological improvement through storage in large man-made ponds or lagoons
  • Reed beds proved a high degree of aerobic biological improvement and can often be used instead of secondary treatment for small communities
• Disinfection using ozone, chlorine, or UV light.
  • Chlorination remains the most common form of wastewater disinfection in North America due it low cost and long term history of effectiveness. The downside is that the chorination of residual organic material can generated 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.
  • UV Light is becoming the most common means of disinfection in the UK because of the concerns about the impacts of chlorine.
  • Ozone 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 any organic material it comes in contact with, destroying many disease-causing microorganisms, with 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. 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 (e.g. solids present in the treated effluent may protect microorganisms from the U.V. radiation).
• 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 of 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.

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.

The biological treatment of phosphorus also involves the design/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) of aluminium (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.

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 which are hostile to these organisms (microbial predation, ultraviolet radiation, etc.) Consequently in cases where the receiving environment provides a high level of dilution to 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 (especially from animal husbandry ) and synthetic materials that mimic hormones in their action can have unpredictable adverse impact on the natural biota and potentially on humans if the water is re-used for drinking water supplies.

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 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 380°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 which may be used to both heat the tank and run engines for other on-site processes. In large treatment plants sufficient energy can be generated in this way to produce electricity for sale. 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 is a bacterial process that runs in the presence of oxygen. Under aerobic (with oxygen) 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 than for anaerobic digestion. However, the operating costs are characteristically much greater for aerobic digestion because of the need to add oxygen to the process. Both the anaerobic and aerobic digestion processes can result in the destruction of disease-causing microorganisms to a sufficient level to allow the resulting digested solids to be safely applied to land or used for agriculture as a fertilizer.
• 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 (similar benefits to peat) for agricultural use.

• The remaining material after digestion can be rich in toxins .Final processing and disposal can be via a number of route with eventual disposal to land as a soil conditioner, land-fill or incineration.
  • Lagooning in drying beds to produce a cake which can be applied to land or incinerated
  • Pressing - where sludge is mechanically filtered , often through cloth screens to produce a firm cake
  • Liquid injection to land or liquid disposal to land-fill
  • Incineration of wastewater solids often requires a lower capital cost but also the need to add sources of fuel to achieve complete combustion. Concerns regarding air emissions are a key issue, along with the high cost of supplemental fuel, making this a less attractive and less commonly constructed means of wastewater solids treatment and disposal.

The choice of wastewater solids 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.

Water reuse

Water shortages throughout the world have many jurisdictions considering means to provide adequate water to communities. The reuse of treated water for a wide range of applications is becoming a more common practice. As an extreme example, in Singapore reuse water is being used as a source of raw water for community drinking water and is being sold in bottled form under the label "New Water". Two residences in the city of Toronto, Canada, use (treated) harvested rainwater for drinking water, and reuse water (i.e. treated wastewater) for all other household water applications including toilet flushing, bathing, showers, laundry, and garden irrigation (Toronto Healthy House). In New Zealand many houses outside of the larger cities and towns routinely rely on rainwater collected from roofs as the only source of water for all household activities. This is almost inevitably the case for the very many Cribs or Bach's (second homes of holiday homes) that exist.

Many large cities using water drawn from Rivers are inevitably re-using sewage effluent discharged upstream. London water had a reputation of having been drunk five times before it arrived at the tap but this an exaggeration. However there are many large towns upstream of London (Oxford, Reading, Swindon Basingstoke etc.) whose sewage discharge is subsequently abstracted to supply London with water.

Considering the increased cost of potable drinking in many drought affected areas of the world, the relatively low cost of treating wastewater for reuse water applications is making this economically attractive for many communities. The City of San Diego, California, and Sydney, Australia, are examples of communities which provide reuse water through dual pipelines to residences, businesses, and industries. In many parts of the world such water is called grey water Costs of reuse water quality treatment can be as low as US$0.50 per cubic metre, in comparison to potable water costs in excess of US$1 per cubic metre.

The largest uses of reuse water internationally is for agricultural and landscape (including golf course) irrigation and deep well injection to replenish declining groundwater supplies.

Administration

In the United States, sewer systems are usually administered on the local level, usually city-wide. These systems, which may operate independently or as a subdivision of a city or other municipal agency, are typically operated as Enterprises, meaning that they produce enough revenues to fund their own activities.

Revenues are usually generated through two charges to customers: connection fees and use charges. Connection fees are charged once to new customers as they connect to the sanitary sewer collection system, and are usually designed to recover capital investments made by the enterprise to serve its customers. Use charges are periodic charges for ongoing use of the system, and are designed to recover operations and maintenance expenses. Both connection fees and use charges are typically proportionate to the amount and strength of wastewater expected to be generated by each customer. Therefore, a single family residence would pay much smaller fees and charges than a food processing plant.

In the UK sewerage and sewage disposal is undertaken by relatively few large companies, including several multi-national companies. The only exception is in Wales where a "not for profit" company is the responsible body but almost all the operational work is sub-contracted by others. Charges to domestic users are based either on the metered volume of incoming water or on the notional value of the property (rateable basis)

Regulation

Sewer systems in the United States are regulated by multiple agencies on the local, state, and federal levels. Federal agencies such as the Environmental Protection Agency (EPA), state agencies such as the Department of Health Services of various states, and local agencies such as regional pollution control boards, all have an interest in the quality of water discharged from sanitary sewer treatment plants. At a minimum, sewer treatment plants must protect the health and welfare of the local population by ensuring that raw or primary treated wastewater does not contaminate the local potable water supply. At a maximum, certain agencies (such as those located in the Lake Tahoe drainage basin) must treat all inflows to tertiary standards, and then pump all treated water out of the drainage basin so that no effluent ever drains to a certain body of water.

Regulating agencies can compel sewer treatment enterprises to construct improvements to their plants by requiring higher standards in effluent quality. If not in compliance with regulations, sewer enterprises may be subject to heavy fines. Regulation is therefore often the driving force behind increasing sewer treatment costs in the United States, and is directly linked to the high cost of constructing or expanding a sewer treatment facility.

In England and Wales sewage disposal is regulated by the Environment Agency with regard to environmental standards and their achievement and by Ofwat in relation to charges and service standards. In Scotland the Scottish Environment Protection Agency fulfils the equivalent that the Environment Agency plays in England and Wales

See also

• Select Society of Sanitary Sludge Shovelers
• sewage
• John Todd

External links

• What happens to all the stuff that goes down the toilet? (http://www.straightdope.com/mailbag/msolidwaste.html) (from The Straight Dope)

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