Understanding the types and quantities of contaminants present in our wastewater helps us predict the requirements of the treatment plant in the future. Improvements to accommodate increased wastewater flows and changing contaminant loads will be a key part of the capital investment and work programme resulting from our Nature Calls project.
Wastewater contaminants originate from residential, commercial and trade waste sources.
Wastewater treatment addresses four major groups of contaminants.
These are undissolved inorganic and organic materials that are suspended in the wastewater and pass through the initial screening process. The main two items are faeces and food. Some of the solids are removed through our sedimentation tanks where they sink to the bottom and are removed for further treatment in our aerobic digester tank. The remaining suspended solids are removed in our lagoons where natural microorganisms and bacteria consume them and transform them into biosolids. A small portion of solids remain in the discharge.
Dissolved contaminants or nutrients
These items include nutrients like phosphorous and nitrogen, and also a range of other contaminants such as metals and inorganic and organic compounds.
Phosphates come from cleaning products, fertilisers and detergents. Nitrates come from protein waste matter and ammonia in urine. These nutrients can contribute to algal blooms in oceans and rivers which overwhelm other plant and animal inhabitants in these environments.
Phosphorus and nitrogen are currently treated and removed in part from the wastewater by biological nutrient removal. At low river flows phosphorus is removed to very low levels by using chemical treatment.
This group includes viruses and bacteria. We measure faecal coliforms as an indicator of the presence of human or animal wastes in the water and while most are harmless, some can cause diseases, such as hepatitis B, gastroenteritis or typhoid.
Micro-organisms are removed at the treatment plant through physical settlement both with and without the use of chemicals while micro-organisms are inactivated using oxidation and UV light which can kill up to 99.9% of these.
Emerging organic contaminants
Emerging organic contaminants (EOCs) are synthetic or naturally occurring chemicals that accumulate in the environment with potential to cause adverse ecological and/or human health effects.
This is an area of ongoing research. Effects of the accumulation of EOCs in the environment are only starting to be understood as detection methods have only recently become available.
Examples include: food additives, pharmaceuticals, and natural and synthetic hormones. The extent to which EOCs are removed through the treatment process is specific to each chemical and the level of treatment. Membrane filtration systems can remove high levels of EOCs.