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Components of Waste Water

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Wastewater is over 99.9% water?

Wastewater is 99.9% water and 0.1% waste.  Everything that goes down the drain or the toilet makes up wastewater.  In order to understand wastewater treatment, it is import ant know the various components that challenge out wastewater treatment processes.

Biochemical Oxygen Demand (BOD)

Biochemical Oxygen Demand (BOD) is a measure of the amount of oxygen that the organic matter in wastewater (food, body wastes, other biodegradable material) “demands” or requires to breakdown into simple more stable compounds.  If too much BOD is released directly into the environment (such as a river, lake, stream or marine water etc.,), the wastewater bacteria will compete with other organisms such as fish, shellfish and other aquatic life, and rob them of the oxygen they need to sustain life.  The goal of wastewater treatment is to reduce the BOD in the treatment process before it reaches area where other organism live. A very typical BOD level in household wastewater is 200 mg/L.  This means that over five days under specific conditions (defined by the Standard Method) one liter of the wastewater will consume 200 mg of oxygen in the process of breaking down.

Total Suspended Solids (TSS)

Total Suspended Solids (TSS) is the amount of solids that are suspended in wastewater and are of a specific size.  When released directly into the surface water environment, suspended solids can be responsible for a variety of problems including smothering habitat, carrying pathogenic organisms, clogging fish gills, smothering fish eggs and more.  When released from a septic tank in high quantities, TSS can prematurely clog leachfields. TSS can be either organic (fine particle of food or feces) or inorganic like silt and clay.  A high TSS value in wastewater is usually associated with a high BOD and indicates that it is to a great extent organic.


Pathogens are organisms or viruses that are capable of causing disease.  The amount of actual pathogens in wastewater will depend on its origin.  When we have a bacterial or viral infection or if we are the host of a certain parasites, we will shed them in our feces and contribute them to the wastewater.  Since it is very difficult to test directly for the majority of pathogens, public health officials use surrogate measures of public health risk called indicator organisms.  Indicator organisms such as Escherichia coli (E. coli) are always present in human waste and are commonly used to suggest the possibility that other pathogenic organisms are present.


Nutrients are compounds that when released into the environment from  wastewater can cause and imbalance of growth of undesirable organism in the environment.  Many “blooms” of noxious algae in freshwater for instance are caused by the release of the nutrient phosphorus.  Some fish kills and habitat modification in marine environments are caused by excessive release of nitrogen with wastewater being a primary source.

Contaminants of Emerging Concern (CEC)

More recently, a  broad class of compounds comprised of pharmaceutical products, personal care products, fire retardants, hormones and others are receiving attention due to their ability to impact aquatic organism at very low (parts per trillion) levels.  Collectively these compounds are called Contaminants of Emerging Concern (CEC).  Perhaps the most documented effect of these compounds is their impact on hormone disruption in fish and other invertebrates.  In some areas extremely skewed sex ratios have been observed and fish populations have become feminized.  Another concern has been cytotoxic drugs taken by individuals undergoing in cancer therapies. Contaminants of Emerging Concern are alternately called Pharmaceuticals and Personal Care Products (PPCPs) or simply micro-contaminants.  Little is known regarding the long term impact on human health caused by many of these contaminants.