The descriptions below are examples of process & technology design, process modification and process innovations, based on findings gathered from in-service systems, works upgrades and project commissioning.

Wastewater Treatment Process & Technology – Process Design, converting a conventional Activated Sludge Process into a Biological Phosphorous Removal (BPR +) system.

The left process design layout shows the original ASP design with two parallel reactors, each with its own PSTs, Anoxic tank, and FSTs. The BPR + design on the right, use all the assets of the original works and shows the modifications from m1 to m9, needed to convert the process.

The additions and modifications are:

m1 – Rerouting the influent into the Anaerobic zone

m2 – Connecting the anaerobic zones of reactor 1 & 2 by extending the channel

m3 – Connecting the anoxic zone with the anaerobic zone via r1 with a new channel

m4 – Rerouting the RAS return to the start of the anoxic zone (4) & rerouting the RAS from the 2nd FST system to the first

m5 – Connecting the aeration lane (L3) in reactor 1, with reactor 2, aeration lane L4

m6 – Anoxic MLSS return sludge pump and elevated distribution chamber

m7 – Additional aeration lane (7) for extended aeration

m8 – Extending the end channels to connect effluent of reactors 1 & 2

m9 – Return MLSS sludge pump and elevated flow chamber for recirculation. Includes the addition of a channel to r2, for recirculation of effluent MLSS to the anoxic zone (4)

The process upgrade is designed to optimise the BPR process and remove nitrate effectively with minimum cost. The design can also be achieved without the additional aeration capacity, but that may potentially reduce the Phosphorous an Ammonia removal capacity.

Changing a conventional ASP system to a BPR + system will always reduce the treatment capacity for BOD and NH3, if there aren’t provision made for additional aeration. This could be costly and may require space for a 20 – 40% larger footprint.  

In the anaerobic zone A, the POAs will use energy from stored PP to accumulate PHA (Poly-hydroxy Alkanoate), release PO4 phosphate.

In the anoxic zone, lane AL1 (Anoxic with acetate), the POAs will use energy from stored PP, and H-Ac to accumulate PHA (Poly-hydroxy Alkanoate), release PO4 phosphate.

In the oxic zone, lanes L4 – L7 (Aerobic with low acetate), the POAs will use energy from stored PHA to accumulate PP from PO4, and for growth. (Accumulate POA biomass)

Removal of the stored poly-phosphate take place with the wasting of sludge, and hence PAOs through the WAS line (11).

The process design innovation shown on the graphic below uses two settling tanks (Sequential Settling) to separate larger, heavier and older sludge floc from the light immature pin-floc, in a continuous flow ASP system. This design is an experimental design based on in-service systems encountered in the field at various locations around the UK. These systems all had the upgrade of humus tanks to newly built FSTs in common, resulting in preferential settling spontaneously occurring when conditions dictated.

In normal operation, the coarse settler removes the heavier floc and maturing grains rapidly, and the sludge is returned normally to the front of the reactor with the coarse RAS line. Smaller and finer sludge remain suspended for longer in the buoyancy force caused by the up-flow. Under these conditions, the system discharges effluent from the coarse settler (6a) to benefit from the improved settling characteristics of the sludge.

To affect preferential settling, the process settings are altered to increase the buoyancy force when required, and finer sludge and light particles are transferred in the overflow to the second settler in the sequence (6b). The fines settler is used to settle out the smaller floc, and the sludge may be wasted through the underflow of the tank (7). If the fines tank effluent quality is sufficient, effluent discharge is switched to the fine settler (8b) during a WAS cycle, or if not, the effluent can be recirculated to the coarse settler and/or aeration reactor with the effluent recirculation system (10).

Repeatedly preferentially selecting only the heaviest and largest floc grains, will result in substantial improvement in settling characteristics, and the densely populated floc grains form large communities of bacteria for BOD, NH4 and phosphorous removal. The larger the grains become, the bigger the anoxic and anaerobic zone toward the middle of the floc. Overly large floc grains are not desirable in continuously fed ASP systems, as the floc grains tend to disperse under low F:M conditions. This could happen when the influent becomes diluted for several months during the rainy season.

The process shown on the design also allows for blending back of the smaller sludge floc, in conditions when the process is required to operate with dilute influent.

Wastewater Treatment – Process design using Dissolved Air Floatation to affect “Preferential Settling” for the Activated Sludge Process, maturing bio-culture size, shape, density and complexity for improved settling characteristics.  

The process design innovation shown on the graphic below uses two solids separation processes, a specifically designed FST, and a Dissolved Air Floation (DAF) system to separate larger, heavier and older sludge floc from the light immature pin-floc, in a continuous flow ASP system. This design is based on in-service systems encountered in the field at various locations around the UK, usually with the DAF added to the process because of poor performance of the FST. The resulting preferential settling that occurs affects the ASP system to mature the sludge floc, beyond the normal calculated values for sludge age.

In normal operation, the FST removes the heavier floc and maturing grains rapidly, and the sludge is returned normally to the front of the reactor with the coarse RAS line. Smaller and finer sludge remain suspended for longer in the buoyancy force caused by the up-flow. Under these conditions, the system discharges effluent from the coarse settler (6a) to benefit from the improved settling characteristics of the sludge.

To affect preferential settling, the process settings are altered to increase the buoyancy force when required, and finer sludge and light particles are transferred in the overflow to the DAF in the sequence (6b). The DAF is used to separate out the smaller floc, and the floated sludge is wasted through the surface skimmer mechanism of the system, as well as the bottom hopper (7). The effluent discharge quality using the DAF is far superior to a normal FST and may be employed for continuous discharge when the process

Repeatedly preferentially selecting only the heaviest and largest floc grains, will result in substantial improvement in settling characteristics, and the densely populated mature floc grains form large communities of bacteria for BOD, NH4 and phosphorous removal. Overly large floc grains are not desirable in continuously fed ASP systems, as the floc grains tend to disperse under low F:M conditions. This could happen when the influent becomes diluted for several months during the rainy season.

Tracking the maturing sludge floc density can be done by monitoring the settling density of the MLSS while keeping the MLSS at the same concentration, with the system usually reaching an equilibrium point for a specific process setup and influent COD concentration. (F:M).

In practice, it is beneficial to adjust the F:M ratio by lowering the MLSS concentration once the SVI reaches a suitable density.

The process shown on the design also allows for maintaining the bio-culture blend to include smaller sludge floc, for better nutrient removal properties and reduced GHG gas emissions performance in the upstream processes.

Often when a WWTW has been given a lower discharge permit for Ammonia, the existing process do not have the aeration capacity to reach the new value comfortably, especially under high load. Additional SAF units are added to the back end of the process, which are often tasked to reduce the ammonia by small amounts like 5, or 8 mg/l.

The problem with this technique, is that a SAF that is seeded with an ammonia feed of 10 mg/l or lower, does not build up nitrification capacity to be able to reduce the ammonia by a great deal. The nitrifiers that do accumulate in the biofilm are few in numbers, and the biofilm growth on the SAF media is thin, due to the low BOD in the SAF feed.

The most important component for an aerated biofilter secondary treatment system, is the Biofilm in attached growth systems, like SAFs, and the Biofloc in suspended growth systems, like ASPs. The system shown below, combines the two types of Bio-technologies to reduce cost and improve performance.

The design innovation shown of the graphic below, integrates four smaller SAF units (11 in the yellow background), two for each aeration system. The SAF units marked -Se, is in the configuration to allow it to seed in with high concentration ammonia and BOD for biofilm formation. The SAF units marked with -D are duty units that are in-service to receive the low ammonia and BOD water at the end of each process.

The process then follows that while the one SAF unit is being seeded in the Se configuration, the other is in-service. When the in-service (-D) unit starts to lose its seed and ammonia removal performance, the units are switched over. Frequent periodic switching between the duty and seeding position may produce an integrated solution for nitrification that reduce ammonia below 1 mg/L, without the use of a constructed wetland.

The process is designed to switch over the two SAF units with a number of channels, flow diversions, chambers and weirs, including four sets of stop valve that are opened and closed. (Red valves are closed, white valves are open)

The four SAF units have additional un-aerated sludge removal in the front of the SAF to prevent the excess solids from entering the aeration reactor with the media. This is connected with the RAS return system via an additional RAS chamber. (12)

Because of the prevalence of combined sewers in the UK wastewater network, permits for the water companies have Full Flow to Treatment (FFT) limits for the treatment requirements. Values for FFT are typical 3 to 4 X Dry Weather Flow (DWF).

To reduce the amount of raw wastewater (untreated) discharges from storm-water overflow, the most cost-effective design upgrade may involve the use of on-site storm-tanks as aeration extension during high flow conditions.

To upgrade the FFT of a works would involve adding flow bypass channels for the inlet, screening and PSTs, feeding diluted raw wastewater directly to the secondary treatment at flow rates in excess of the current FFT (upgrade from 4 X DWF to 8 X DWF).

The increased flow would require additional FSTs (2X capacity, yellow block 5), to return the biomass solids to the reactor. The additional FST capacity only become operational under excess flow conditions (flows above current FFT).

The increased flow and increased RAS return cause significant decrease in aeration time in the reactor (HRT), resulting in possible loss of nutrient removal capacity (BOD, NH4). Flow is redirected to the aerated storm tanks (8,9) via the flow path at 4. The storm tanks are only operational when excess flows exceed the nutrient removal capacity of the main reactor, monitored from the effluent discharge quality. The biomass used for treatment in the tanks are diluted from the MLSS in the main reactor. (3)

The storm tanks may still discharge directly to the final effluent (10) under extreme conditions, but the discharge would be significantly diluted, nearly complete treated water. The normal discharge pathway from the aerated storm-tanks involves biomass recovery in the FSTs (5, 6).

Modern membrane technology can out-perform conventional technology by a huge margin, but it comes at a cost and require diligent management to protect the membranes.

ASP systems can effectively be fitted with Ultra Filtration systems in place of final settlement tanks, with the ultra membranes rejecting the floc, dispersed bacteria and possible pathogens, with 100% efficiency. Because of this, in a treatment process, such as shown on the image above, the ultra filters are a key to protect the Reverse Osmosis membranes from bio-fouling and bio-film growth.

In the system shown, the RO systems functions in parrallel with the constructed wetland, with RO permeate passing directly into the water treatment reservoir for feed to the process or drinking water plant. The wetland receives the RO brine which is concentrated with alkalinity, ammonia, phosphate and nitrate rejected by the RO membrane. The wetland plant flourish on the rich nutrient water and if the quality permits, the water discharge, including the mineral content can be recycled into the reservoir, ready for treatment.

Converting a contact stabilisation process to a EBPR process require minor changes to the works layout and technology. The summary of the changes are listed for the on-site construction and engineering as:

1.    Re-route inlet to Anaerobic zone (I)

2.   Disable all aeration in Anaerobic and Anoxic zones (Grey)

3.   Re-route scum trap air-lift & discharge point (From (Sd) to Aerobic Zone (Md))

4.   Re-route Scum trap air-lift discharge at Anoxic zone weir (From (E) to Aerobic Zone (Md)).

5.   Raise emergency weir over hight (E).

6.   Add submerged sludge pump (Suspended from a chain on an adjustable cross strut) to raise MLSS from Anoxic zone (From (M) to Aerobic Zone (Md)). This pump may also serve as the additional mixer to re-suspend MLSS that will settle at the weir (E & M).

7.  Reroute RAS using mechanical pumps (discharge point (R)). This involves removing the airlift system and using positive displacement pumps mounted on top of existing manifold. (RR)

8.   Reroute SAS to the sludge holding tanks (automated valve (S)). Using the same pump as the RAS, SAS may be moved through a time-controlled valve off the RAS line. (S)

9.   Install submerged sludge pump with flexible discharge (Recirc A). To pinpoint the pump’s collection point on the edge of the Anoxic zone and return sludge to the start of the fermentation zone. (A) Both the pump’s installation point and discharge point (Ad) should be adjustable. (Suspended from a chain on an adjustable cross strut)

10.   Replace old aeration distribution valves with fully functioning ones.

11.   Adapt aeration to release pressure and lower DO in Aeration zone Ae1 – Ae5.

The changes to the works re-design were driven by applied chemistry methodology for Biological Phosphorous Removal, and the following notes summarise the design drivers.

Technical notes on generating the bio-chemistry applied for an EBPR process in a contact stabilisation tank arrangement:

• BPR rely on the Anaerobic/Aerobic cycling of Biomass (MLSS) between three different metabolic states that PAOs (Polyphosphate Accumulation Organisms) are capable of. (All three zones Anaerobic/Aerobic/Anoxic)

• When POAs are at the end of the accumulation phase (Ae05) they have taken up the phosphate available in the wastewater and removal of the phosphate happen with the removal of surplus sludge (SAS).

• During the Anaerobic/Fermentation stage of the process (Grey), substrate containing short chain fatty acids (e.g. Acetate) are added to the MLSS (at point (I)) recirculated from upstream of the Anoxic zone (from point (A)). POAs use their accumulated PP (Poly-phosphate) stores to metabolise the substrate, releasing orthophosphate into the water. POAs utilise the substrate to form PHAs (Polyhydroxyalkanoates) as an internally stored substrate for use in unfavourable conditions. PAOs are advantaged over GOAs under these conditions as there is no DO or NO3 as electron acceptors available. The anaerobic zone will have several dead zones at the bottom of the tank where solids will drop out over time. These dead zones will further advantage the POAs as substrate limitations may stimulate them to remain in an accumulation metabolic state, resulting in an increase in numbers.

• In the anoxic zone at the end of the anaerobic zone, the nitrates (NO3) present from the RAS discharge point is denitrified with the catabolism of the GOAs and the high levels of VFAs entering the zone from the fermentation zone.

• The boundary zone between anaerobic and anoxic zone should be created by the net flow of water from the inlet and recirc from point (A), theoretically positioned just outside the reach of Nitrate Oxygen (Which crucially should not be recirculated to the inlet).

• The RAS discharge point should be sub-surface but high in the water column. The MLSS pump at position (M) should be at the bottom of the water column to create the longest pathway back down to the bottom of the weir/baffle. This is to enhance nitrification using the MLSS submerged pump to re-suspend part of the biosolids near the RAS discharge point.

• In the anoxic zone GAOs will start to dominate metabolism by using nitrate oxygen and the PAOs will adopt a different metabolic strategy, by using the available VFAs to form more PHA stores.

• The anoxic/aerobic zone starts where the scum traps discharge adding the air from the airlift mechanism. The zone has high levels of VFAs and GAOs dominate competition for both substate and electron acceptor. Under these conditions, POAs will accumulate PHB and release phosphate into the wastewater.

• When VFAs are lower towards later aerobic zones (Ae03 – Ae05) POAs will accumulate rapidly increasing in numbers and utilising orthophosphate from the water to build stores of polyphosphate up.

• The baffle in the contact stabilisation process between Ae03 and Ae04 serves no purpose if the process is converted to a EBPR process. This baffle would have been a very good option to divide the anoxic zone off from the fermentation zone, an option that should be considered in future design strategy. Additional design options to utilise the baffle in its current position (to change the zone to a walled off anoxic zone) will require re-routing of the weir/FST feed plumbing of the current system.

•  After the end of aerobic zone 05 the phosphate in the water have been taken up by the POAs, the MLSS is separated in the FST and the POAs are wasted in the SAS line to the sludge tank, effectively removing the phosphate from the system.

Activated Sludge process to suppress Nitrous Oxide (N2O) emissions from WWT

Although Nitrous Oxide emissions from WWTW are considered a bit of a “nothing burger”, suppressing NO2 emissions will lead to proper treatment and removal of BOD, NH4, NO3 , and further prevention of acidic water discharges.

Nitrous Oxide emissions may be kept to a minimum if the process Ammonia, Nitrate, Dissolved Oxygen and Alkalinity concentrations are carefully managed not to cause excessive stress on the bio-culture.

 This ASP system has been designed with a buffer/dilution, and anoxic tank system in which ammonium concentrations can be lowered with recycled effluent water, and stored momentarily to even out the load. At the same time, anoxia in these tanks will assist de-nitrification as a good supply of short chain VFAs are available from the settled crude.

This process would cost 30% more than a conventional process to build (CAPEX) with operating cost (OPEX) remaining the same or being marginally more, for vastly improved performance.

The buffer/dilution tanks are designed as FSTs, as they have to handle return activated sludge containing nitrate and the heterotrophic bio-culture required for de-nitrification. 

Nitrous Oxide emissions from WWT processes are a concern due to their greenhouse gas potential. Although records of Nitrous Oxide emissions have been shown to minimal, there are conditions in certain WWTw, where these emissions are unacceptably high. In the next section we look at the origin of N2O from the process, and how to re-design the process to minimise them.

The attached diagram shows an Activated Sludge process with an additional tank before the bio-reactor, that can be used to optimised de-nitrification. RAS is recycled into the Anoxic/Dilution tank, and there is an additional FE recirculation system to be used as a further part recirculation loop for nitrate.

To generate favourable conditions in the de-nitrification tank, there should be no dissolved oxygen left in the returned sludge and make-up water. Most of the nitrate that are left in the RAS and the recirculated FE effluent, would serve as the electron acceptor during the de-nitrification process during anoxic conditions.

Bacterial di-nitrification take place via four enzyme catalysed reactions as an electron transport chain, with the different nitrogen-oxygen bonds accepting the electrons sequentially, until the terminal electron acceptor is released as diatomic nitrogen gas N2. In order for a Nitrate molecule to be completely reduced to Nitrogen requires the enzymatic donation of 5 electrons, in total 10 electrons to form 1 diatomic nitrogen.

If we compare the bond energy in nitric oxide, a single triple bond, with the bond energy for the two Nitrous Oxide double bonds, the high activation energy required to overcome the formation of N2O become apparent. Nitrous oxide is also a gas, and the next step in the reduction requires a further 2 electrons from the enzyme catalysing the reduction. The high activation energies dictate a steady supply of electrons from the carbon oxidation reactions in the cell cytoplasm, as well as the presence of adequate alkalinity to maintain the proton balance. If these two conditions are not at an optimum, the reduction to nitrogen is slowed, and nitrous gas may easily escape and emit from the water surface.

The design of the process as shown on the diagram, allow for a stringent anoxic environment, with high quantities of nitrate as electron acceptors present, as well as the highest possible concentration of SVFAs for fast fermentation and quick supply of electrons to drive the reduction. Alkalinity dosing can easily be affected by making sure the pH of the reaction water remain basic or at the optimum pH of 7.5.

Wastewater Treatment – Activated Sludge System design modification from a conventional design, to a Step Feed system with membrane filtration.

The attached schematic below shows an ASP system design modification to change a conventional system to a Step Feed system, capable of Biological Phosphorous Removal (BPR) and optimised denitrification. The system was designed in response to new, lower permit discharge limits for phosphorus, nitrate, ammonium and suspended solids. The process discharged effluent into an ecologically sensitive water body and protected area. The alternative option was a constructed wetland, but the site had limited space, and extra land would have to be purchased.

To affect denitrification in the end lane of the process, the most significant modification added to the conventional ASP was the Ultra Filtration units, which can be seen in the schematic with a yellow background – modification 6 (m6). On conventional AS processes, denitrification at the end of the aeration basin results in very poor settling characteristics in the final settler, due to entrained nitrogen gas trapped in the floc.

The step feed modification in this design, marked modification 3 (m3), feeds extra fermented influent from the Anaerobic Zone with high acetate content, to the connecting channel (m2), as step feed 2 (red arrow), at point 6 into lane 4 (L4). Lane 4, now with a high acetate concentration, effectively denitrifies the treatment stream and starts to oxidise the newly added ammonium towards the end of the lane.

Modifications m4 and m5 are extensions of the effluent channel from the end of lane 6, channelling the full treatment stream under anoxic conditions (middle to end of lane 6 not aerated), to be equally distributed between the two ultra-filtration installations.

Membrane ultrafiltration is very effective for separating secondary solids regardless of their settlability, and the secret of this design resides in the flow percentage split of the RAS (to Basin 2/Basin 1), as well as the flow % split of the influent, between system 2/system 1.

If the process is properly balanced, the Anoxic zone (3, AO) in front of the aeration basin 1, becomes anaerobic within minutes due to the low nitrate load being returned with the RAS, optimising the BPR capabilities further.

A step-feed process accelerates denitrification in the second anoxic zone, which leads to reduced Nitrous Oxide emissions from the process and from the receiving water.

This content was extracted from a Specialised Topics learning module from learnwastewater@resenviro.com.