The descriptions below are process & technology Information Communication output, designed to inform personnel with responsibilities operating, optimizing and managing wastewater treatment processes. The bulletins are short communications authored to address known chemistry and bio-chemistry aspects of the process needed achieve process outcomes.

Tracking the mass of solids in the wastewater influent is a vital activity when operating, optimising or problem solving an Activated Sludge process. Using population equivalent (PE) estimates or modelled data for process design, is hardly effective or even vaguely applicable to an in-service operated system.

The best measurement for tracking the mass flow through the process to evaluate overall performance is Total Solids (TS), TS (10) / TS (1) = % removal. Environmental permits use COD for tracking the organic solids load through the process with COD (10) / COD (1) = % removal of all organic fractions, BOSup + IOSup + BOSet + IOSet.

COD process modelling as proposed in academic ASP models, is so grossly incorrect that it is not even worth wasting time with it on an in-service system. ASP modelling data only applies to benchtop processes (with moderate accuracy), where the influent solids mass feed is controlled to be 100% biodegradable.

The biodegradable fractions as can be seen below, are the most important fractions to consider, as most of the metabolic supply of the bio-culture, to provide energy and cell growth (bio-accumulation), come from the BOD in dissolved and suspended fraction. The particulate BOD fraction (BOSet), which is not measurable with a BOD analysis, provide some metabolic supply over days, but this depends on particulate complexity, and is not measurable by a BOD measurement.

BPR is a challenging process with detailed knowledge of biochemistry processes and micro-biology required by attending technical support personnel.

Recovery of the stored poly-phosphate takes place with the wasting of sludge, and hence PAOs through the WAS line, shown at point 9 on the graphic below.

The design shown below include several innovations specifically to optimise the process for BPR recovery in the polyphosphate containing biosolids removed from the FSTs.

The major design changes to a conventional BPR system are:

• Counterflow and underflow bio-culture solids recycling

• Subsurface influent points to optimise process conditions

• Variable tank levels for volume and HRT adjustments

• Low level tank overflow systems

The physical design innovations are necessary to be able to manipulate the conditions to be most favourable for the Poly-phosphate Accumulating Organisms, POAs (4, 6, 8). The intricate counterflow of concentrated solids allow for efficient cycling of bio-culture solids between optimised aerobic, and optimised anaerobic conditions (3, 7, 10, 11, 12, 13, 14, 15). The process can also be adjusted by using sludge blankets and tank levels to extend and reduce the volume capacity of the anaerobic or anoxic zones, according to process requirements for optimisation of phosphate removal (11, 12, 13).

BPR rely on the principle that Poly-phosphate Accumulating Organisms (POAs) will remove ortho-phosphate from the water, and accumulate Poly-phosphate (PP) inside the cell bodies during aerobic/starvation conditions. When the bacteria are then cycled back to Anaerobic conditions with available VFAs & Acetate, the POAs use the stored PP to release energy (no O2 for respiration), and store a reduced carbon polymer alkanoate (PHA) for future starvation periods.

In the anaerobic zone, the POAs will use energy from stored PP to accumulate PHA (Poly-hydroxyalkanoate), and release PO4 phosphate in the bulk water.

In the anoxic zone, and in the presence of SVFAs, the POAs will use energy from stored PP, and H-Ac to accumulate PHA, and release phosphate.

In the aerobic zone and with SFVAs in short supply, the POAs will use energy from stored PHA to accumulate PP from PO4, and for cell growth (Accumulate POA biomass).

The process design innovation shown on the graphic below, uses two settling tanks to separate larger, heavier and older sludge floc from the light immature pin-floc, in a continuous flow system.

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 RAS line. Smaller and finer sludge remain suspended for longer in the buoyancy force caused by the up-flow. Under these conditions to process discharge effluent from the coarse settler, to benefit from the improved settling characteristics of the sludge.

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. The fines settler is used to settle out the smaller floc and the sludge may be wasted through the underflow of the tank. If the quality permits, effluent discharge is switched to the fine settler during a WAS cycle, or if not, the effluent is recirculated to the coarse settler and/or aeration reactor with the effluent recirculation system.

Repeatedly 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.

The process also allows for blend back of the smaller sludge floc for when the process conditions require to operate with dilute influent.

Mass flux through a FST is a calculable parameter used when designing an ASP system or Mass Flux mathematical models are often used for developing knowledge and skills in academic courses and WWT learning. The predictive aspirations of the mathematical models do not have much application in the operation and management of in-service systems.

It is a very good exercise to study the parameters used in Mass Flux mathematical models and compare them to the measured chemical values available for your process management. It is also very obvious to see why the system is not able to conform with the restrictions of the model, and would differ from system to system and with the considerable drift of the variables due to changing conditions.

The important variable parameters to consider when optimising the mass flux in your FSTs are:

FST inflow value => The process influent flow + the RAS return flow

Solids load – FST inflow value X MLSS measured concentration

Volumetric load – The net flow volume in the FST, FST inflow – RAS flow = FE flow

FST rise rate – The upward flow in the FST in m/hr (red arrows on graphic), FE flow / available volume in FST

Sludge Volume Index – SVI measurement on Biomass culture sample (MLSS)

RAS concentration – May be measured directly or estimated from SVI

In scenario 1 on the graphic below, the incoming flow representing the volumetric and solids load, is low, the underflow (RAS return) set to the optimum point, where the FST maintains the highest mass flux efficiency possible. Under these conditions the sludge blanket will remain at a determined level in the tank, and the RAS concentration will be near the highest it can be for the MLSS concentration at the time.

In scenario 2, the incoming flow (volumetric and solids load) is high, with the underflow adjusted to maintain the blanket level in the tank, thereby maintaining the concentration of the RAS near the highest concentration possible. In this scenario, there will be a slight deterioration of the solids in the overflow (final effluent) due to the increased rise rate in the FST. This obviously indicates that under high flow there is also a marginal drift away from the previous optimum mass flux point.

In scenario 3, the underflow is not sufficient to maintain the mass flux near the optimum point and the blanket will rise, the solids in the overflow will increase rapidly until it ultimately leads to solids washout to the final effluent discharge. Several factors may be responsible for this, these being, exceeding the loading rates, poor sludge condition or the required RAS return rate exceeds the maximum rate possible.

Scenario 4 depicts the conditions where the underflow is too rapid and the concentration of the RAS decrease, with the drop in blanket level. This scenario may lead to inefficient settling affecting the mass flux through the underflow, and increasing the solids load in the weir overflow. Maintaining the concentration of the RAS at or near the optimum point is an effective way to manage the FST mass flux under varying conditions.