Powerstep

In addition, aeration demand can be more precisely adjusted to the actual requirements of the biological process with frequency-controlled blowers and online aeration control based on automatic sensors for dissolved oxygen and ammonia.

Enhancing the extraction of carbon into sewage sludge in the WWTP process will increase the potential biogas yield and decrease efforts for treatment, thus improving the overall energy profile of the WWTP considerably. Enhanced carbon extraction can be realised by advanced primary treatment combining chemical dosing with sedimentation, filtration or flotation processes to separate most of the particulate matter. After enhanced carbon extraction for energy recovery, downstream biological nitrogen removal has to be optimised to safely fulfil effluent quality targets.

This excess heat can be partially valorised with processes that turn heat into electricity (“heat-to-power”), drawing on excess or low-grade heat of the WWTP and increasing electricity production at the plant. Available technologies for heat-to-power include steam or organic rankine cycle units, or thermoelectric generators. The economic feasibility of these units also depends on the actual seasonal heat profile of the WWTP with varying amount of excess heat in summer and winter operation.

Several options are available to increase this conversion ratio, produce more biogas from sludge and reduce final sludge cake for disposal. Digestion process can be operated in two dedicated stages or in cascading design, optimising process efficiency and capacity utilisation. Sludge can also be treated thermally or chemically before digestion to improve hydrolysis and increase degradation. Improved digestion can also have an effect on the return load from sludge dewatering, which has to be taken into account in an overall concept for optimised sludge management.

If enhanced carbon extraction is targeted in primary treatment, a strategy for low-carbon N removal (i.e. without external carbon source) is required. Advanced control of WWTP operation based on online sensors and effluent quality can help to optimise the utilisation of residual carbon, thus enabling a sufficient degree of N removal with the remaining carbon source. Another potential approach involves a switch of the biological process to promote a combination of nitritation (ammonia to nitrite) and anammox bacteria, which can convert nitrite and ammonia to N2 without using a carbon source. This process has already been realised in full-scale for sidestream treatment where high ammonia concentration and high temperature provide favourable conditions for these bacteria.

The application of nitritation/anammox in mainstream (“mainstream anammox”) is more challenging due to less favourable conditions such as low temperature and high dilution, but has been tested and optimised in industrial pilot scale to fulfil effluent quality requirements. This form of low-carbon N removal will enable the operation of new WWTP schemes where enhanced carbon extraction delivers a high yield of biogas and mainstream anammox as secondary treatment can still remove nitrogen below target values.

Suitable processes for N recovery from this sidestream are mainly based on physico-chemical processes such as air stripping or membrane stripping. Both processes rely on the transfer of gaseous NH3 into sulfuric acid, resulting in a liquid diammonium sulfate solution which can be sold as fertiliser.

Biological processes include conventional nitrification combined with denitrification, but this process requires high amount of oxygen and a carbon source. Deammonification applies two steps of nitritation and anammox, converting ammonia to gaseous nitrogen without the need of an external carbon source. Eventually, nitrogen conversion can also be stopped at nitrite (nitritation) to recycle the chemically bound oxygen to the mainline, using mainstream carbon for N removal.

This process describes the conversion of CO₂ and hydrogen into methane, which can be realised by catalytic or biological processes. Hydrogen can be produced with an electrolyser operated during times of low electricity prices, producing H₂ and O₂ from water. While H₂ is used for methanation, O₂ and excess heat of the electrolyser can be used on-site to cover the internal demand of oxygen for the activated sludge system or to provide heat to the WWTP. The use of a P2G plant enables the integration of WWTP infrastructure into the dynamic energy market of the future and optimises economic revenues from biogas production.

Different options are available for sidestream N removal, such as biological nitrification and denitrification, nitritation, deammonification, or stripping. The latter process even enables N recycling, fixing the nitrogen in a liquid or solid product which can then be sold as N fertiliser.