The first stage of a WWTP removes a large fraction of particulate matter from the influent wastewater after mechanical pre-treatment for sand and grid removal. This primary treatment removes fine particles by physico-chemical treatment, typically by simple sedimentation in a primary clarifier. More advanced primary treatment uses chemical dosing to improve particle separation with sedimentation, filtration or flotation.
The secondary treatment step in a WWTP consists of a biological process to remove dissolved organics and nutrients. Typically, microbial biomass feeds on the dissolved substances in an activated sludge system, converting them to gases (CO2, N2) or fixing them in sewage sludge.
Tertiary treatment is applied to polish WWTP effluent quality before discharging it into receiving waters and have a targeted removal of specific groups of substances. Multiple treatment principles are available to remove residual solids, dissolved organic and inorganic substances, or reduce microbial contamination.
Effluent water from sludge dewatering after digestion contains high amounts of dissolved nitrogen in the form of ammonia, increasing the nitrogen load to the mainstream WWTP with recycling of the sludge water. This nitrogen can be removed or even recovered into a nitrogen product in a separate treatment of this sidestream, reducing the nitrogen return load to the WWTP.
Biogas produced at a WWTP can be converted into biomethane (> 95% CH4) by separating CO2 and impurities or converting CO2 into methane using a biological or catalytic methanation process. Biomethane can then be directly injected into the public gas grid, providing stable revenues for the operator due to high storage capacity of the grid and stable gas prices.
Anaerobic digestion of sewage sludge generates biogas, which can be valorised on-site at the WWTP. Biogas can be utilised in combined heat and power plants to produce electricity and heat for self-supply or for sale to external customers. Other forms of valorisation include gas boilers or microgas turbines.
A WWTP requires electricity for water treatment, but it can also generate electricity from biogas produced in digestion of sewage sludge. This dynamic profile of electricity supply and demand can be coupled with the highly dynamic market of electricity in the public grid, enabling an integrated operation of this “smart grid” to minimise energy costs and optimise revenues for the WWTP operator.
Electricity is the main driver for energy costs at a WWTP, and it is mostly used for aeration of biological treatment steps. Electricity consumption can be significantly reduced by increasing the efficiency of the aeration system or by reducing the oxygen needs of the biological stage. Enhanced carbon extraction in primary treatment and low energy nitrogen removal can both reduce aeration demand and related electricity consumption, enabling savings in operational cost.
Energy production at WWTPs mainly builds on the anaerobic digestion of sewage sludge to produce biogas, which is then converted to electricity and heat, or sold as biomethane to the grid. Different options are available today to increase the amount of biogas produced at a WWTP: maximising the amount of primary sludge, optimising the digestion process, or using external co-substrates to make use of the full digestor capacity.
Energy can be produced in various forms at a WWTP, e.g. biogas, electricity, or heat. Some parts of this energy may not be fully utilised, such as biogas which is flared, or waste heat that is not used. Making use of this excess energy can improve the energetic profile of the WWTP.
The integrated energy market of today has a dynamic price structure and multiple levels of engagement for market players. The rising share of renewable energy (e.g. wind, solar) leads to fluctuating energy prices and new challenges in grid load management, both providing opportunities for WWTP operators to reduce their energy costs and obtain maximum revenue from selling energy to the market.
Municipal wastewater contains valuable nutrients such as nitrogen, phosphorus, and other trace elements. Those nutrients are accumulated in sludge or sludge water, and can be partially recovered with physical or chemical treatment. Recovered nutrients can be sold to farmers to close the loop of nutrient management and substitute mineral fertiliser production.