13 More Lessons Learnt As A Trainee Wastewater Process Engineer

1. Balancing Flows Within WRRFs

• Using flow balancing to attenuate the flow to the secondary treatment process to the range of 80 - 120% of Average Dry Weather Flow (ADWF) helps in terms of reducing impacts of diurnal ammonia and nitrate breakthrough.
• Mixing is to be provided; jet mixers are a good maintainable option. Odor control covers are required, but achieving the required air changes per hour is expensive (large odour control unit).
• Flow balancing might be the best solution for sidestream management (instead of treating the sidestream using shortcut nitrogen removal technologies) – depending on other process conditions and effluent quality requirements.
• For WRRFs without flow balancing, peak oxygen demand is usually 1.3 or 1.35 times the average demand.? Dry weather flow balancing reduces this diurnal peaking factor (a lot).? However, as a dry weather flow balancing only attenuates dry weather flows, the aeration system still needs to consider the peak diurnal demand when the balance tank is full (or out of service for maintenance).
• Swing zones are included in some designs to reduce ammonia breakthrough during the diurnal peak flow period.? As flow balancing assists with diurnal ammonia breakthrough, a swing zone intended for this purpose is not worth it.? Alternatively, swing zones are sometimes included to manage seasonal variations (i.e. low operating temperatures, uncertainties in nitrification rates, managing maintenance events etc.).? A swing zone with this purpose can still have a role to play under these circumstances.

2. ‘Excess’ Biological Phosphorus Removal (EBPR)

• Phosphorus Accumulating Organisms (PAOs) are different to ordinary heterotrophs, taking up readily biodegradable COD (as VFAs) and converting it into Poly Hydroxy Alkanoates (PHAs).
• Glycogen Accumulating Organisms (GAOs) compete with PAOs and can therefore inhibit the EBPR process.
• Utilizing plug flow reactor configuration is a good design approach for anaerobic and aerobic zones.
• Shorter sludge age favors EBPR.
• Mass fraction: 10 - 15% of entire bioreactor for anaerobic zone ideal.
• Depending on influent TP, in a well-designed and operated EBPR based BNR, you can potentially see ~0.1 ppm TP in treated effluent.

3. Phosphorus Removal Using Chemicals

• Alum and / or iron metal salts are most preferred. But alum has a slight edge over iron salts as the former does not feature under ‘dangerous goods’ category. Also, iron salts can cause concrete corrosion and impact UV lamps downstream (if used for disinfection).
• Always perform alkalinity balance for BNRs with chemical dosing (simulators can be used for this activity).
• A good approach is to dose the chosen chemical at multiple locations (i.e. to the secondary process, and then prior to downstream filters) . This is because multiple point dosing affects the molar ratio, sludge generation, efficacy of phosphorus removal and the downstream processes.

4. Achieving Low TN In BNR Effluent

• Meeting TN<5/3 ppm is possible without dosing external carbon. Many WRRFs have made provisions for substrate dosing but have not utilized it consistently.
• For internal mixed liquor recycle (‘A’ recycle), submersible pumps are not as efficient as axial flow pumps. That’s why older conventional design textbooks limit the internal recycle to be 5 - 6 times the influent flow rate (higher rates could not be provided efficiently).
• But with axial flow pumps, higher internal recycle rates are possible. Spacing between the pumps is critical to their mechanical design.? Pumps can be staggered to achieve the required offset between adjacent units.
• Rate of denitrification is controlled by rate of food or substrate available to the microbial mass. There are three denitrification rates namely K1 (with readily biodegradable COD), K2 (with slowly biodegradable COD), K3 (substrate release from cell lysis used for denitrification).
• K3 is about ten times slower as compared to K1, which means the anoxic zone size would need to be higher. An alternative option is to dose an external carbon in the anoxic zone.
• If you are using an external carbon source in the secondary anoxic zone, the K3 value should be replaced with K belonging to the external carbon source (should be somewhere near K1).
• External carbon options are methanol, ethanol, acetate, molasses etc. But the carbon selection is influenced by health and safety considerations in its storage and handling, impurities, costs (both CAPEX and OPEX).
• Methanol has a slower K value as compared to the sugar solution which means slower utilization rate (marginally). In addition, sugar solution does not have major flammability or explosion concerns.? Critically, however, methanol is utilized by a distinct group of organisms which must be established before the dosed substrate can be processed.? To this end, rapid increases in methanol dose rates can be an issue (as there will be a lag before the methanol utilizing organisms can grow to consume the material).

5. Design Considerations for Membrane Bioreactors (MBR)

• MBRs offer lower footprint (civil) as compared to Conventional Activated Sludge (CAS) process. But the savings in civil costs mostly get traded for mechanical and electrical costs (membrane scouring blower, pipework, higher Recycled Activated Sludge [RAS] flow, Clean in Place [CIP] equipment etc).
• For 3 x ADWF condition, MBRs are attractive. Above that, not so much. Therefore, peak wet weather flows are a key determining factor for the technology application.
• Sustainable MLSS in membrane tank: 10g/l (long term), 12 g/l (for shorter peaks)
• For a 4 stage Bardenpho configuration, it is recommended to have a dedicated secondary aeration zone prior to the membrane tank. After dosing an external carbon to the secondary anoxic zone, you do not want accidental filtration of any left-over carbon through the membranes.
• PUMP TO configuration allows for better level control in bioreactors.
• Extracellular Polymeric Substances (EPS) can be controlled in MBRs.? The rule of thumb by membrane suppliers is that if the system is fully nitrifying, the sludge age is long enough to reduce concerns over excessive EPS. Membrane fouling is more problematic if you lose nitrification owing to EPS.
• Bleed the CIP waste slowly back to headworks after collecting the concentrated waste in a separate tank. Alternatively, after a sodium hypochlorite (NaOCl) CIP, Sodium Bisulphite (SBS) can be dosed to quench the NaOCl and then it can be slowly fed back to the headworks of the WRRF.
• During low flow period, it is recommended to recycle the RAS flow (from membrane tank) via the fine screens to capture more screenings.

6. Oxidation Ditch A Viable Technology Option

• A good choice for nitrogen removal as the internal recycle rates can be massive (up to 150 times the influent flow rate) to meet TN<3 ppm.
• With reactor compartmentalization (an upfront anaerobic zone), TP<1 ppm is achievable.
• Higher recycle rates are possible with axial flow pumps in compartmentalized systems. They typically have low head (0.3 - 0.6m). With a common wall between the end of the aerobic zone and the start of the secondary anoxic zone, these pumps can be placed on the shared wall, doing away with the need for pipework (and the associated head losses).
• In oxidation ditches, varying the aeration rate (Dissolved Oxygen [DO] set point) can be used to adjust the mass fractions (anaerobic, anoxic, and aerobic), enabling nutrient removal performance to be optimized.
• Circulating velocity in ditches to be maintained as 0.25 - 0.3 m/s (minimum) to keep the biomass in suspension.
• The construction challenge with ditches is the round edges (and the need for more footprint).

7. Feedback on Aerobic Granular Sludge (AGS)

• Currently, there are three full scale installations in Australia.
• Design comprises of a complex feed manifold at bottom and a fixed decant launder at the top of the tank.
• Owing to granule formation, the settling velocity is high. This can translate to higher TSS in treated effluent.
• Earlier designs required dedicated mixers, but current designs do not need them.
• This technology is more suitable for brownfield WRRFs (less influent TSS) as compared to greenfield WRRFs (higher influent TSS).
• Design MLSS: 7 - 8 g/l (similar to MBRs)
• A fundamental truth of microbiology: organisms that grow fast, die fast (applicable to AGS). Organisms that grow slow, die slow (eg: anaerobic ammonia oxidizing bacteria). For some reason, if the AGS process is upset, you need to have a contingency plan in place to transfer similar granular sludge from elsewhere.
• In terms of nutrient removal performance, TN<5 ppm is easily achievable. TN<3 ppm is more difficult especially if an external carbon must be dosed. Good biological phosphorus removal has been observed but one of the installations in Australia is also dosing alum for removal.

8. Be Careful with Aeration Design

• There are two important considerations for the aeration system design i.e., the blower and the diffuser grid.
• The designer must hold concurrent talks with the blower manufacturer and the diffuser aeration grid manufacturer.
• Diffuser manufacturers work with SOTR, not AOTR.
• A good approach is to reach out to the diffuser manufacturer (to work out SOTR, SOTE%) first and then to the blower manufacturer (to optimize Nm3 / hour).

REMEMBER: There is difference between Sm3 / hour (at 20 degree Celsius) and Nm3 / hour (at 20 degree Celsius) which can account for a difference of 6 - 7% in the design air flow rate.

• Fine bubble diffusers do not work well in MBR bioreactors having solids concentration in the range of 1.3 - 1.5% (13 - 15 g/l).
• As the aeration diffusers in BNRs foul with organics and / or inorganics over time, it is important that an inspection and cleaning are carried out every 12 - 18 months.
• The aeration grid lifting system also needs to be utilized at about this frequency to mitigate the impact of screenings which can accumulate on the grids (adding to their weights) and obstruct the lifting guides and cables.
• An intuition, and experience is required to select the Alpha Fouling factor as the measured value can be different to the design. This chosen factor is also dependent on the chosen BNR technology, diffuser type etc.

9. Considerations For Secondary Clarifiers (SC)

• If sufficient land footprint is available in a WRRF, SCs are a good option.
• Irrespective of design capacity, as a minimum, two clarifiers are a must in the design. If there is no redundancy, there is no biological treatment process.
• During Peak Wet Weather Flows (PWWF), all clarifiers must be in service. During Peak Dry Weather Flows (PDWF), a clarifier can be taken out of service.
• It is recommended to consider 80th or 90th %ile of many years’ monitoring results for selecting the settleability and pollutant loads applied to designing clarifiers (do not use the median values alone).
• SCs are designed based on PWWF and DSVI.
• The solid particles interact with each other and settle in the hindered settling phase, and this becomes the governing design and operational criteria.
• When pumping RAS out of clarifier, it should be flow paced to the plant influent rate.
• There are two types of failure that occur in SCs namely:

1. Clarification failure (occurs when the upflow velocity > settling velocity)

2. Thickening failure (if RAS recycle ratio isn’t maintained, then the mixed liquor overflows the launders)

• A flocculation skirt provided as part of the clarifier inlet structure helps with avoiding clarification failure.
• Though covering clarifiers can help to an extent of avoiding algae growth in the launder section, you might observe growth of freshwater sponges instead.

10. Dangers of PFAS The New Problem Child

• PFAS stands for per and poly fluoroalkyl substances (PFAS). They have unique properties which makes their degradation and treatment challenging.
• Put simply, PFAS are manmade chemicals, and are used as coating agents to bind particles. They are used in cookware, paints, shampoos, dental floss, and fire extinguishers (used during training drills within army and air bases).
• As the micro organisms break down organics in BNR, PFAS are released. Therefore, you might see an increase (30 - 40%) in PFAS concentration from inlet to effluent. Sunlight can help to release PFAS compounds.
• Treatment technologies that are an option to treat PFAS in liquid stream are: Advanced Oxidation, Reverse Osmosis (RO), Ozone with Biological Activated Carbon (BAC), Ion Exchange and Granular Activated Carbon (GAC).
• PFAS is not destroyed during pyrolysis. However, an option of dealing with PFAS in biosolids involves burning them at >800?C.

11. Handling Biosolids Within WRRFs

• As the TN and TP discharge limits get stringent for liquid stream, the associated biosolids processing becomes challenging. Phosphorus cannot be destroyed and ends up in biosolids.
• With biosolids, the bigger challenge is process selection rather than process design.
• More pathogens end up in sludge as compared to liquid stream.
• Some of the heavy metals (lead, zinc, mercury) bind with sulphide within raw wastewater in sewers. In the BNRs, the sulphide gets converted to sulphate. The metals are released and end up in sludge which cannot be treated.
• For the final biosolids product (compost or drying), there needs to be a viable market (balance between supply and demand).

12. The Rise of Computational Fluid Dynamics (CFD)

• Process engineers should become familiar with CFD modelling as it can be used for design of processes within WRRFs (Anaerobic Digesters [AD] mixing, vortex grit chambers, BNR mixing, flow splitters etc).
• With the rise of CFD, the conventional civil hydraulic design is set to become redundant over time.
• As sludge is non Newtonian fluid, its viscosity is an important consideration (within ADs) and can be modelled using CFD models.
• An available open source model for CFD can be explored further here.

13. General Advice and Technical Books Recommendation

During breaks, in conversation with the course instructors, I learnt:

• Our career is shaped by answering the question: "what do I want to do?"
• For a process engineer, the decision to join an organization must primarily be based on "who will I be working alongside with".
• For equipment and components within WRRFs, useful information is available on manufacturers website and in datasheets.
• Select publications by Water Research Foundation (WRF) and conference papers (eg: Water Environment Federation, Australian Water Association) have good information that can be utilized for projects.
• Following are some of the useful technical publications recommended by the course instructors: