The following section describes microbiological, physical and chemical parameters typically used to describe wastewater quality.
Ammonia
Nitrogen (along with phosphorous and carbon) are essential nutrients for growth. When discharged to the aquatic environment, these nutrients can lead to the growth of undesirable aquatic life. In a wastewater treatment plant, ammonia is normally oxidised to nitrites and then to nitrates. A low ammonia concentration in final treated wastewater effluent therefore normally indicates that effective nitrification has occurred within the wastewater treatment process. Considering the above, ammonia measured in drinking-water may indicate the presence of untreated sewage. If the ammonia concentration exceeds the required limits, intervention is required to rectify the situation (e.g. optimise operation at the treatment plant).
Chemical Oxygen Demand
Biodegradable organics are principally composed of proteins, carbohydrates and fats, and are commonly measured in terms of Chemical Oxygen Demand (COD). If discharged untreated to the environment, their biological stabilisation can lead to depleted oxygen levels and the development of septic conditions. If the COD exceeds the required limits, intervention is required to rectify the situation (e.g. optimise operation at the treatment plant).
Electrical Conductivity
Electrical Conductivity (EC) is the measure of the ease with which water conducts electricity and gives an indication of the total dissolved salt (TDS) content of the water. Health effects related to EC occur only at levels above about 370 mS/m. Adverse effects may include disturbance of salt and water balance in infants, heart patients, individuals with high blood pressure, and renal disease. The EC measurement (in mS/m) can also be used to estimate the TDS (in mg/L). If the EC exceeds the required limits, intervention is required to rectify the situation (e.g. ensure source protection, ensure treatment plant can effectively reduce EC/TDS, optimise operation at the treatment plant).
Feacal Coliforms
Faecal coliform bacteria are found in water wherever the water is contaminated with faecal waste of human or animal origin. Faecal coliforms are primarily used to indicate the presence of bacterial pathogens such as Salmonella spp., Shigella spp., Vibrio cholerae, Campylobacter jejuni, Campylobacter coli, Yersinia enterocolitica and pathogenic E. coli. These organisms can be transmitted via the faecal/oral route by contaminated or poorly treated water and may cause diseases such as gastroenteritis, salmonellosis, dysentery, cholera and typhoid fever. The risks of being infected correlates with the level of contamination of the water and the amount of contaminated water consumed. Higher concentrations of faecal coliforms in water will indicate a higher risk of contracting waterborne disease, even if small amounts of water are consumed. Also to note is that for any treated wastewater discharges, 0% failure with regards to specified limits for faecal coliforms is required. Any bacteriological failure with regards to faecal coliforms can therefore be considered as a direct indication of risk to health. If the faecal coliform count exceeds the required limits, intervention is required to rectify the situation (e.g. optimise disinfection).
Free Chlorine Residual
Free chlorine residual is an indication of the efficiency of the disinfection process and is thus a rapid indicator of the probable microbiological safety or otherwise of the treated water. Absence of residual chlorine means either that the water was not treated with chlorine, or that insufficient chlorine was used to successfully disinfect the water. Where the untreated water contains pathogenic microorganisms, the absence of free residual chlorine indicates that there is a risk of microbial infection. If the free chlorine residual does not meet the required limits, intervention is required to rectify the situation (e.g. optimise disinfection).
Nitrates and Nitrites
High concentrations of nitrates/nitrites in drinking-water are generally associated with methemoglobinaemia in infants ("blue-baby syndrome"). In a wastewater treatment plant, ammonia is normally oxidised to nitrites and then to nitrates. If the nitrate/nitrite concentration exceeds the required limits, intervention is required to rectify the situation (e.g. ensure source protection, ensure that the treatment plant can effectively remove nitrate/nitrite, optimise operation at the treatment plant).
Phosphates/Phosphorous
Eutrophication is the natural phenomenon that results due to the gradual enrichment of a water body with nutrients (e.g. phosphates). However, accelerated eutrophication resulting from unnatural excessive discharge of nutrients to water systems is undesirable. If the phosphate/phosphorous concentration exceeds the required limits, intervention is required to rectify the situation (e.g. ensure source protection, ensure that the treatment plant can effectively treat phosphates, optimise operation at the treatment plant).
pH
The pH of a solution is given by the expression: pH = -log10[H+], where [H+] is the hydrogen ion concentration. At pH less than 7, water is acidic, while at pH greater than 7 water is alkaline. A direct relationship between the pH of drinking water and human health effects is difficult, if not impossible to establish since pH is very closely associated with other aspects of water quality. The taste of water, its corrosivity and the solubility and speciation of metal ions are all influenced by pH. At low pH, water may taste sour, while at high pH water tastes bitter or soapy. To minimise corrosion, it is recommended that optimium pH levels are maintained. If the pH value does not meet the required limits shown above, intervention is required to rectify the situation (e.g. optimise stabilisation).
Sodium Adsorption Ratio
The sodium adsorption ratio is used to predict potential infiltration problems. If the infiltration is greatly reduced it may be impossible to supply the crops or landscape plants with sufficient water for vigorous growth.
Suspended Solids
Suspended solids can lead to the development of sludge deposits and anaerobic conditions when untreated wastewater is discharged to the aquatic environment. If the suspended solids concentration exceeds the required limits, intervention is required to rectify the situation (e.g. ensure source protection, optimise operation at the treatment plant).
Total Kjeldahl Nitrogen
Raw sewage contains quite high concentrations of urea and amines and proteinaceous compounds which break into ammonia on standing. The Total Kjeldahl Nitrogen (TKN) is the method that measures all these forms of nitrogen and gives the total nitrogen present in the sewage or wastewater.