• 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.

2.1.4 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 plant s 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.

2.1.5 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.

3 Administration

3.1 Regulation