Powerstep

I want to reduce electricity consumption of my WWTP

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.

Electricity consumption at a WWTP is mainly driven by the demand of blowers for aeration, supplying oxygen to the biological treatment stage. This major factor in operating costs of a WWTP can be reduced by installing a highly efficient aeration system, thus improving the oxygen transfer. Another approach is to enhance carbon extraction in the primary treatment, thus decreasing the organic load on the biological process and lower oxygen needs. This enhanced primary treatment can be coupled with advanced control or innovative low-energy nitrogen removal processes to still enable biological elimination of nitrogen without adding an external carbon source. Separate treatment of highly loaded sludge dewatering effluent can also reduce the overall electricity consumption of the WWTP by providing efficient treatment of the return load.

Read more

Read less

Efficient aeration

Efficient aeration

Aeration of the biological stage is the major driver for energy demand at a WWTP, typically accounting for 40-60% of the total electricity demand. Hence, improving the efficiency of aeration is a key measure to reduce electricity consumption and related costs of WWTP operation. Aeration efficiency can be increased by improving the transfer of oxygen from the gas into the liquid phase, e.g. by replacing mechanical surface aerators or upgrading bubble aeration with new diffusors for fine-bubble aeration.

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.

Read more

Read less

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.

Read more

Read less

Membrane aerated biofilm reactor

This technology was not part of the POWERSTEP project.

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.

Read more

Read less

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.

Read more

Read less

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.

Read more

Read less

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).

Read more

Read less

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.

Read more

Read less

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.

Read more

Read less

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.

Read more

Read less

Advanced control

After advanced primary treatment to remove a large fraction of COD, the remaining wastewater has a low COD/N ratio. Hence, optimal usage of the remaining carbon is vital to reach stable denitrification capacity in the biological step. This can be reached in SBR-type WWTPs by applying advanced control strategies at the WWTP process control system: in standard operation, an adapted feeding and aeration regime of the SBR can optimise the denitrification capacity, while dedicated backup strategies triggered by an increase in nitrate concentrations in the SBR can help to keep effluent TN values below the target.

For standard operation this means that SBR is mainly fed in denitrification phases, and aeration is controlled by oxygen depletion rate instead of time. Backup strategies include reducing COD extraction in primary treatment (reduction of chemical dosing, bypassing primary treatment) or utilising other available carbon sources (supernatant of sludge thickening, acetate dosing). For advanced control strategies, online monitoring of nitrate is required, while online monitoring of ammonium is recommended as well.

Read more

Read less

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.

Read more

Read less

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.

Read more

Read less

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.

Read more

Read less

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.

Read more

Read less

Sidestream N removal

SIDESTREAM N REMOVAL OR RECOVERY

Sludge dewatering effluent contains high concentrations of ammonia, originating from anaerobic digestion of organic matter. This sidestream is usually recycled to the inlet of the WWTP, increasing the load to the treatment plant and related energy demand. It can also be treated in a separate process for N removal, which can be more efficient than mainstream treatment due to high concentrations and elevated temperature of the sludge water.

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.

Read more

Read less

Technologies

Air stripping

Sludge liquor from dewatering of digested sludge contains a high load of nitrogen, which is recycled back to the inlet of the WWTP. This return load usually contributes 10-20% of the total nitrogen load of the plant. It can be reduced by dedicated N removal in this sidestream, which can be realised by different processes. Apart from biological processes which eliminate the nitrogen into the atmosphere, ammonia can also be stripped in suitable conditions of high pH and temperature and then be recovered in an acidic solution, which can finally be used as liquid fertiliser.

Air stripping is a proven technology which utilises ambient air to strip ammonia in a first stripper column, which is then recovered into sulfuric acid in a second column. Air stripping is typically operated at 70°C and pH 10 in columns filled with carrier material to enhance surface area for gas-liquid transfer. Air stripping is most cost-effective at high ammonia concentrations and is limited to a maximum elimination of nitrogen due to the chemical equilibrium of ammonia.

Read more

Read less

Membrane stripping

Sludge liquor from sludge dewatering is usually heavily loaded with nitrogen and contributes significantly to the nitrogen load of the WWTP. Beside biological processes for sidestream N removal (e.g. deammonification) which can have issues in process stability, N can also be removed by air, steam or membrane stripping in a physico-chemical process, thus enabling the recovery of nitrogen as a valuable liquid fertiliser (ammonium sulfate). Membrane stripping represents a compact and cost-effective method to recover the nitrogen in sludge liquors by gas-permeable membranes. To convert nitrogen into free ammonia and enable stripping, pH and/or temperature of the sludge liquor have to be increased.

Optimising pH control (e.g. by CO2 stripping combined with some caustic) and temperature conditions minimises the efforts in heat and chemical costs, and allows cost-effective operation of membrane stripping. Pre-treatment of the sludge liquor with coagulation/filtration also has to guarantee particle-free influent to the membrane in order to protect the membrane module from mechanical damage and clogging.

Read more

Read less