I want to increase energy production of my WWTP

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.

WWTPs can produce energy from the organic matter coming with the raw wastewater, mainly by generating primary or biological sludge during treatment which is then converted to biogas in anaerobic digestion. The amount of biogas produced depends on the total amount of generated sludge, but also on the degradability of this material in the digester. New concepts for WWTPs target an enhanced extraction of organic matter in the primary stage, producing more primary sludge with high biogas yields and reducing the organic load to the secondary biological treatment. In addition, thermal or chemical pre-treatment of the sludge can enhance conversion rates of sludge into biogas and reduce final sludge cake and related disposal costs. Digestion modes can also be changed into a two-stage or cascading scheme to improve biogas production. If free capacity is available in the digester, the addition of external co-substrates (e.g. food waste, fats) will yield additional biogas that can be valorised at the WWTP.

Enhanced C extraction

Enhanced C extraction

Municipal wastewater contains a variety of organic substances, and the embodied carbon represents a potential source of chemical energy. Traditional WWTPs separate a fraction of this carbon (up to 50%) into primary and excess sludge, while the remaining part is converted to CO2 in biological treatment by microbial biomass. The latter part requires a high amount of electricity for oxygen supply, and wastes the potential energy contained in this part of the organic matter.

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.

Technologies

Chemically enhanced primary treatment

The primary treatment stage in a WWTP usually consists of a sedimentation tank or settler, where particulate matter is separated from influent wastewater by simple gravity-driven sedimentation. Due to their high specific weight, particles settle to the ground and are collected there as primary sludge, normally operating at retention times of 0.5 to 2 hours. To enhance the separation of smaller particles and reduce required retention times, coagulation and/or flocculation can be added upstream of the settler, which is then called chemically enhanced primary treatment (CEPT). Dosing of metal salts and/or polymer aggregates smaller particles and colloidal matter into larger aggregates, which can then be separated by gravity.

Chemical dosing can be combined with lamellar settlers which provide a high surface area for settling and thus allow a more compact design. The combination of coagulation and/or flocculation tanks and lamella settler can also be enhanced with sand as a nucleation agent of flocs, which is then called “ballasted sedimentation”. The sand is separated from primary sludge by hydrocyclone before being recycled to the flocculation tank. CEPT allows a higher extraction of COD in primary treatment (up to 50-70%) than simple settlers and thus can be a first step for an energy-efficient WWTP scheme.

Dissolved air flotation

Dissolved air flotation (DAF) is a water treatment process that clarifies wastewater by the removal of suspended matter such as oil or solids. The removal is achieved by dissolving air in the wastewater under pressure and then releasing the air at atmospheric pressure in a flotation tank basin. The released air forms tiny bubbles which adhere to the suspended matter causing the suspended matter to float to the surface of the water where it can then be removed by a skimming device. DAF can be operated with upstream dosing of coagulants or flocculants to agglomerate particulate matter and enhance elimination efficiency.

The process can also be combined with lamellar settlers for a compact footprint. DAF is highly suitable for eliminating fatty or oily compounds in wastewater which usually originate from industrial discharge (e.g. food processing, slaughterhouse). For municipal wastewater, DAF can be used for enhanced carbon extraction as primary treatment step in a WWTP, especially if industrial contribution with floatable organics is substantial.

Microscreen (Drum+Discfilter)

Microscreen technology (drum- or disc-filter) employs woven cloth filter elements installed on the periphery of a drum or disc structure, and utilises an inside-out flow pattern. The filter works by gravity with a few cm of water pressure gradient and is robustly designed with few moving parts to ensure long life and low maintenance costs.

Water to be treated flows by gravity into the filter via the centre drum. The media mounted on the partially-submerged drum separates solids from the water. The filtered water flows through the media into the collection tank. Once solids have accumulated on the filter and a critical head loss is achieved, the media is cleaned by a counter-current backwash system. The filters are in continuous operation even during backwash and high solids events.

Coagulants and flocculants can be added upstream the filter unit in order to improve the filterability of the particles. The dose and type of chemicals applied also have an impact on the pollutant removal efficiencies that can be achieved in the microscreens. Such a feature allows for precise particle removal up to the values required by the user by careful control of the chemical dose (mg-chemical added/TSS to be removed).

Two-stage activated sludge

The 2-stage treatment concept for municipal WWTPs provides energetic advantages in term of reducing energy demand for aeration and enhancing the energy gain from biogas utilisation in CHP. The concept relies on a high COD-loaded activated sludge tank in the 1st stage where a higher percentage of COD - as compared to single-stage WWTP - is extracted from the wastewater stream with the surplus sludge.

In the 2nd stage which is operated at higher sludge age, nitrification and denitrification take place. Nitrogen removal can be optimised and adjusted to varying operating conditions by bypassing a part of influent wastewater in the 2nd stage for increasing the COD availability for denitrification or by returning a part of the nitrate rich WWTP effluent back to the anoxic zone of the 1st stage. The advantages of 2-stage treatment concepts could be confirmed at several large size WWTPs.

The energetic advantage of the 2-stage treatment configuration can be further increased by integrating a nitritation process to treat the ammonium-rich sludge dewatering effluent in sidestream. The treated wastewater stream can be then reintroduced in the 1st stage, promoting denitritation of the produced nitrite instead of denitrification. The saved COD load is then available for enhanced biogas production in the digester or for improving TN removal at plants with unfavourable TN/COD influent composition. In addition, the implementation of sidestream nitritation decreases operating costs of a 2 stage WWTP due to the better conditions for ammonium oxidation in sidestream than in mainstream (higher oxygen transfer rate).

Whereas sidestream nitritation combined with denitritation in mainstream is nearly energetically equivalent to sidestream anammox (oxygen demand in both cases ~ 1.5 gO2/gNremoved), nitritation exhibits a much more stable operation and lower investment and operation costs for process control.

Improved digestion

Improved digestion

Sewage sludge is often stabilised by anaerobic digestion, reducing the organic matter and converting it into biogas. This process is usually operated in a digestor, where sludge is heated to mesophilic conditions (35-38°C) and digested for up to 20 days. With this process setup, a conversion of up to 50% of organic matter is typically achieved.

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.

Technologies

Cascade digestion

Sewage sludge is usually digested at a WWTP in one-stage mesophilic systems. This simple design is robust and easy to operate, but the biological conversion of organic matter into biogas is limited within normal retention times (15-20 days). In addition, the heterogenic matrix of sewage sludge and non-optimal reactor design leads to improper mixing in the digestor, so that parts of the reactor are not active (“dead volume”) and actual retention time of the sludge may be lowered, leading to incomplete degradation.

This problem can be mitigated by operating two reactors in series in a cascade design, where the sludge from the top of digestor 1 is fed by gravity to the top of digestor 2. Statistical variability of retention time can be lowered by the defined transfer of sludge from one reactor to another, so that the overall retention time is more close to the targeted value. Cascade digestion proved to increase biological degradation of sewage sludge without requiring larger tank volume. Existing digestors can thus be operated in cascading mode to make better use of the available infrastructure.

Thermal hydrolysis

This technology was not part of the POWERSTEP project.

Low-carbon N removal

Low-carbon N removal

The removal of nitrogen at a WWTP is realised in biological processes, where the dissolved ammonia is converted to gaseous N2 by microbial activity. The conventional process consists of the steps of nitrification and denitrification, converting ammonia to nitrate and then nitrate to gaseous N2. This process requires a high amount of oxygen for nitrification, and a carbon source for denitrification. Both aspects have a high impact on the overall energy balance, as oxygen supply needs electricity and carbon consumed during denitrification is not available for biogas production.

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.

Technologies

Activated sludge

The process of activated sludge has been developed more than 100 years ago. It is based on the removal of dissolved substances from wastewater using suspended microbial biomass (“activated sludge”). Using the metabolism of the microbial biomass, organic matter and other pollutants such as nitrogen and phosphorus are eliminated to atmosphere or transferred into the biomass. This process requires large amounts of oxygen, which is supplied to the biomass by introducing air into the system. Aeration can be realised by e.g. bubble diffusors or surface aerators, and aeration efficiency is a major cost factor for WWTP operation.

While most of the biological sludge is separated from purified water in a final sedimentation tank (clarifier) and recirculated to the activated sludge process, a fraction of the surplus sludge is separated as excess sludge from the treatment process. There are multiple configurations for activated sludge systems to enable the enrichment of specific microbial groups for elimination of nitrogen and phosphorus. Sludge age, biomass concentration and oxygen conditions are major parameters to be adjusted in an activated sludge systems to guarantee the desired capacity of the treatment process.

Deammonification

Deammonification is the process of nitritation-anammox and is a shortcut in the conventional nitrification/denitrification process. Deammonification is performed in two steps: aerobic nitritation, where slightly more than half of the ammonia is oxidised to nitrite by ammonia oxidising bacteria (AOB) and anoxic ammonia oxidation, where ammonia and nitrite are converted by anammox bacteria to nitrogen gas. Compared to conventional nitrogen removal process, no organic carbon is required for nitrogen removal and large savings of oxygen and energy demand can be achieved.

Deammonification process can be configured in one or two reactors. Performing deammonification in two reactors, where aerobic nitritation and anoxic ammonium oxidation are performed separately, gives better possibilities for optimising the growth conditions for AOB and anammox bacteria. On the other hand, deammonification in one single reactor benefits from generally lower investment costs and lower demand for operational maintenance. To allow simultaneous nitritation and anammox process, one stage deammonification is carried out in biofilms, where the biofilm can grow on carriers, or be in granules or flocs. In the biofilm, the AOB are enriched in the outer layer of the biofilm and the anammox bacteria in the anoxic inner layer of the biofilm. Specific operational conditions (pH, temperature, oxygen level) are maintained within the reactor to allow the process specific bacteria to grow in the biofilm.

Deammonification for side-stream treatment is a state of art technology, where high ammonia concentration with low COD concentration are typical characteristics of the side-stream water from dewatering of digested sludge. This reject water is usually sent back to the main treatment line, where the treatment of this ammonia load through conventional nitrification/denitrification can be quite cost-intensive. The use of deammonification process on side-stream water can dramatically reduce the nitrogen load on the existing biological treatment line. It is also a way to treat a part of the nitrogen at a low energy. In addition, since it allows a reduction of the nitrogen load on the main treatment line, it is also a solution to upgrade an overloaded existing wastewater treatment plant at a low cost.

Mainstream Anammox (1-stage)

Deammonification is the process of nitritation-anammox and is a shortcut in the conventional nitrification/denitrification process. Deammonification is performed in two steps: aerobic nitritation, where slightly more than half of the ammonia is oxidised to nitrite by ammonia oxidising bacteria (AOB) and anoxic ammonia oxidation, where ammonia and nitrite are converted by anammox bacteria to nitrogen gas. Compared to conventional nitrogen removal process, no organic carbon is required for nitrogen removal and large savings of oxygen and energy demand can be achieved. The slow-growing bacteria in deammonification are generally grown in biofilms on carriers (as part of the MBBR technology) or in granules and/or flocs.

Today, deammonification technologies for side-stream treatment are state-of-the-art. However, the application of deammonification on mainstream municipal effluent faces several challenges, due to the low temperature, low ammonia concentration and higher COD/NH4-N ratios, which result in competition between the different bacterial consortiums (AOB, Nitrite Oxidising Bacteria (NOB), anammox bacteria and heterotrophs (OHO)), and the requirements for an efficient NOB repression strategy.

One approach for implementation of deammonification in the mainstream is the one-stage Integrated Fixed Film Activated Sludge (IFAS) process, which consists of a Moving Bed Biofilm Reactor (MBBR) with the addition of an external settler for sludge retention. With this configuration, anammox bacteria preferentially grow in the biofilm on the carriers while the aerobic AOB tend to grow in the suspended sludge. The integration of activated sludge and biofilm is an effective approach to improve single-stage biofilm deammonification processes, as suspended growth is capable of enriching nitrifiers and improving the rate of nitritation at low DO levels due to less limitation on mass transfer, while carriers with anammox biomass are easily and securely retained in the reactor.

For mainstream deammonification applications, the IFAS configuration provides fast and robust physical separation between anammox-rich biofilm carriers and nitrifier-rich suspended sludge, which allows for easy control of the sludge age in the system and therefore efficient selective wash-out of NOB while retaining anammox. The IFAS configuration also has the advantage of being more tolerant towards soluble biodegradable COD (sbCOD), making it possible to handle an influent with a relatively high sbCOD/NH4-N ratio. Additionally, the IFAS configuration can be readily retrofitted into existing activated sludge systems.

Mainstream Anammox (2-stage)

Deammonification is the process of nitritation-anammox and is a shortcut in the conventional nitrification/denitrification process. Deammonification is performed in two steps: aerobic nitritation, where slightly more than half of the ammonia is oxidised to nitrite by ammonia oxidising bacteria (AOB) and anoxic ammonia oxidation, where ammonia and nitrite are converted by anammox bacteria to nitrogen gas. Compared to conventional nitrogen removal process, no organic carbon is required for nitrogen removal and large savings of oxygen and energy demand can be achieved. The slow-growing bacteria in deammonification are generally grown in biofilms on carriers (as part of the MBBR technology) or in granules and/or flocs.

Today, deammonification technologies for side-stream treatment are state of the art. However, the application of deammonification on mainstream municipal effluent faces several challenges, due to the low temperature, low ammonia concentration and higher COD/NH4-N ratios, which result in competition between the different bacterial consortiums (AOB, Nitrite Oxidising Bacteria (NOB), anammox bacteria and heterotrophs (OHO)), and the requirements for an efficient NOB repression strategy.

One approach for implementation of deammonification in the mainstream is the two-stage Moving Bed Biofilm Reactor (MBBR) configuration. With the MBBR, the biomass is growing on carriers which are easily and securely retained in the reactor. Hence, the two-stage MBBR system for deammonification enables a compact solution with a high accumulation of nitrifying biomass.

With the two-stage configuration for deammonification, ideal conditions for each of the desired bacteria group can be achieved in separate MBBRs. In the first MBBR, aeration ensures a high DO to achieve efficient nitritation with AOB biofilm, while in the second, a mechanically mixed and anoxic stage, nitrite and ammonia are converted to nitrogen gas through the anammox pathway, without any competition for substrates from oxidising bacteria or heterotrophs. NOB suppression is achieved in the nitritation stage by maintaining thin biofilms and periodically exposing the biomass to strongly suppressing conditions, such as switching the feed from mainstream wastewater to high-strength and high-temperature side-stream water. Hence, NOB can be supressed in the aerated zone without affecting the anammox. Thin biofilms are ensured in the nitritation stage by using the AnoxKTM Z carrier with biofilm thickness control.

Moving Bed Bioreactor (MBBR)

The Moving Bed Biofilm Reactor (MBBR) technology is based on the biofilm principle, with an active biofilm growing on small specially designed plastic carriers. The carriers are designed to provide a large protected surface area for the biofilm to grow and are kept in suspension in the reactor by aeration and/or mixing.

Due to the high biomass content in the biofilms, the MBBR process has a small footprint and high volumetric removal efficiency. The carriers are retained in the reactor by sieves, ensuring that the critical biomass will remain in the reactor also at low temperatures and high hydraulic flows. When needed, the MBBR can be staged in series to promote enrichment of different groups of bacteria dependent on reactor conditions and substrate availability. The MBBR works both for aerobic, anoxic and anaerobic applications.

The MBBR technology has been successfully applied since the 1980´s, treating both industrial and municipal wastewaters. Generally, the MBBR is applied for the biological removal of carbon (through heterotrophic respiration) and nitrogen (through nitrification-denitrification), although the long retention of biomass in the MBBR also makes it advantageous for deammonification, and for the removal of some toxic compounds and micropollutants. The MBBR can also be used in combination with existing activated sludge systems in a hybrid system.