Wastewater
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Wastewater is any water that has been adversely affected in quality by any anthropogenic influence. It therefore includes liquid waste discharged from domestic houses, industrial, agricultural or commercial processes. It does not include rain-water uncontaminated by human activities.
There is a wide range of wastewaters Template:Ref label and an equally wide range of technologies and techniques for mitigating the impacts of wastewaters on the receiving environment.
Contents |
Wastewater types
Industrial
- Organic - bio-degradable - includes abattoirs, creameries, ice-cream manufacture
- Organic - non bio-degradable or difficult to treat - for example Pharmaceutical or Pesticide manufacturing
- Inorganic - for example metal working industry
- extreme pH - acid/alkali manufacturing, metal plating
- Toxic - e.g. metal plating, cyanide production, pesticide manufaturing
- Solids and Emulsions - e.g. Paper manufacturing, food stuffs, lubricating and hydraulic oil manufacture
- agricultural drainage - direct and diffuse
Surface Runoff
- Highway drainage
- Storm drains
- industrial site drainage
- Black water - surface water contaminated by sewage
Domestic drainage
- Sewage
- Cesspit leakage
- Septic tank discharge
- Sewage treatment plant discharge
- Grey water also known as sullage water - water from household functions such as washing dishes, laundry or bath water
Wastewater Quality Indicators
Wastewater quality indicators such as the Biochemical oxygen demand (BOD) and the Chemical oxygen demand (COD) are tests that measures the oxygen demand of a wastewater. The oxygen demand of a wastewater is in turn a measure of whether or not that wastewater will have a significant adverse effect upon fish or upon aquatic plant life.
Any oxidizable material present in a natural waterway or in an industrial wastewater will be oxidized both by biochemical (bacterial) or chemical processes. The result is that the oxygen content of the water will be decreased. Basically, the reaction for biochemical oxidation may be written as:
- Oxidizable material + bacteria + nutrient + O2 → CO2 + H2O + oxidized inorganics such as NO3− or SO42−
Oxygen consumption by reducing chemicals such as sulfides and nitrites is typified as follows:
- S2− + 2 O2 → SO42−
- NO2− + ½ O2 → NO3−</b>
Since all natural waterways contain bacteria and nutrient, almost any waste compounds introduced into such waterways will initiate biochemical reactions (such as shown above). Those biochemical reactions create what is measured in the laboratory as the Biochemical or Biological Oxygen Demand (BOD).
Oxidizable chemicals (such as reducing chemicals) introduced into a natural water will similarly initiate chemical reactions (such as shown above). Those chemical reactions create what is measured in the laboratory as the Chemical Oxygen Demand (COD).
Both the BOD and COD tests are a measure of the relative oxygen-depletion effect of a waste contaminant. Both have been widely adopted as a measure of pollution effect. The BOD test measures the oxygen demand of biodegradable pollutants whereas the COD test measures the oxygen demand of biogradable pollutants plus the oxygen demand of non-biodegradable oxidizable pollutants.
The so-called 5-day BOD measures the amount of oxygen consumed by biochemical oxidation of waste contaminants in a 5-day period. The total amount of oxygen consumed when the biochemical reaction is allowed to proceed to completion is called the Ultimate BOD. The Ultimate BOD is too time consuming, so the 5-day BOD has almost universally been adopted as a measure of relative pollution effect.
There are also many different COD tests. Perhaps, the most common is the 4-hour COD.
It should be emphasized that there is no generalized correlation between the 5-day BOD and the Ultimate BOD. Likewise, there is no generalized correlation between BOD and COD. It is possible to develop such correlations for a specific waste contaminant in a specific wastewater stream ... but such correlations cannot be generalized for use with any other waste contaminants or wastewater streams.
The laboratory test procedures for the determining the above oxygen demands are detailed in the following sections of the "Standard Methods For the Examination Of Water and Wastewater" available at www.standardmethods.org:
- 5-day BOD and Ultimate BOD: Sections 5210B and 5210C
- COD: Section 5220
Treatment
Template:Details There are numerous processes that can be used to clean up waste waters depending on the type and extent of contamination. Most wastewater is treated in industrial-scale wastewater treatment plants (WWTPs) which may include physical, chemical and biological treatment processes. However, the use of septic tanks is widespread in rural areas, serving up to one quarter of the homes in the U.S. The most important aerobic treatment system is the activated sludge process, based on the maintenance and recirculation of a complex biomass composed by micro-organisms able to degrade the organic matter carried in the wastewater. Anaerobic processes are widely applied in the treatment of industrial wastewaters and biological sludge. Some wastewater may be highly treated and reused as reclaimed water. For some waste waters ecological approaches using reed bed systems such as constructed wetlands may be appropriate. Modern systems include tertiary treatment by micro filtration or synthetic membranes. After membrane filtration, the treated wastewater is indistinguishable from waters of natural origin of drinking quality. Nitrates can be removed from wastewater by microbial denitrification, for which a small amount of methanol is typically added to provide the bacteria with a source of carbon. Ozone Waste Water Treatment is also growing in popularity, and requires the use of an ozone generator (See: http://www.biozone.com/ozone_generators.html ), which decontaminates the water as Ozone bubbles percolate through the tank.
Reuse
Treated wastewater can be reused as drinking water (Singapore), in industry (cooling towers), in artificial recharge of aquifers, in agriculture (70% of Israel's irrigated agriculture is based on highly purified wastewater) and in the rehabilitation of natural ecosystems (Florida's Everglades).
Further reading
External links
- Industrial Pollution Information from the Coastal Ocean Institute, Woods Hole Oceanographic Institution