Q: How is sludge age used to simplify control of the activated sludge process?
A: The idea that something can be simplified has always driven innovations in technology. The activated sludge (A/S) wastewater treatment process, which has been around since 1914, is no exception. Operators of A/S know that process control is critical due to ongoing demands for lower costs and higher performance. Process control is successful if the A/S biomass consistently settles, compacts, and flocculates well, yielding high effluent quality.
Process control seeks biomass stability, a state of equilibrium that balances biomass growth, metabolism, and decay. Influent wastewater variability presents a continuous challenge to biomass stability. In response, operators have three adjustable process control variables available:
- Waste activated sludge (WAS) flow rate
- Return activated sludge (RAS) flow rate
- Air/oxygen supply
Of these three variables, the WAS flow rate is most directly related to biomass health as defined by specific growth rate. Biomass specific growth rate (µ) is related to sludge age (θc) as follows:
µ = 1/ θc + γ + b, where γ and b are specific death and decay rates
The operator has direct control of specific growth rate by manipulating sludge age through wasting:
θc = biomass solids inventory
solids wasting rate
As sludge age is increased, specific growth rate decreases, and vice versa. Maintaining a constant sludge age leads to biomass stability. Sludge age is held constant at a set wasting rate, allowing the A/S system to self-regulate, adjusting the biomass inventory in response to influent loading. Attempting to maintain a constant biomass inventory (as measured by MLSS or MLVSS) results in frequent changes to sludge age due to influent variability, and the operator becomes a source of variability.
Sludge age control is simplified by wasting mixed liquor instead of RAS, making the wasting rate independent of clarifier operation and RAS concentration. The wasting rate becomes a hydraulic expression:
θc = Aeration Vol x MLSS = Aeration Vol
WAS flow x MLSS WAS flow
For example, if you have 5 MG of aeration volume, a WAS flow rate of 0.5 MGD would result in a 10-day sludge age. The operator would simply set a constant WAS flow rate at 350 gpm and allow the A/S system to naturally adjust MLSS up or down in response to influent loadings. The sludge age is known at all times with no need for solids testing results. Depending on effluent limits, the sludge age can be adjusted seasonally to accommodate mixed liquor temperature and nitrification requirements.
Q: Every winter we get excessive foaming in our anaerobic digesters. Our digester loadings seem fine, and the digester pH and volatile acid/alkalinity ratio is acceptable as well. What might be the cause and what can be done?
A: Wastewater treatment plants in the upper Midwest, especially those utilizing biological nutrient removal, are susceptible to proliferation of Microthrix parvicella in the winter.
Q: When using iron salts for phosphorus removal, how can we reduce or eliminate vivianite problems on the belt press?
- Skip Poster, City of Portage
A: Vivianite is a hard, dense blue-gray precipitate of hydrated ferrous phosphate (Fe3(PO4)2•8H2O). The dewatering anaerobically digested sludge (combined primary and waste activated) often results in scale formation caused by the precipitating of vivianite or struvite (magnesium ammonium phosphate). The release of biologically retained phosphorus under the anaerobic conditions in the digester causes the digesting sludge to reach a saturated condition with respect to phosphate. Exposure to air and turbulence during sludge dewatering strips out carbon dioxide, resulting in a slight rise in the pH, which is enough to lead to the phosphate precipitation. Typically, the best strategy to diminish the scale nuisance is to reduce the phosphate concentration in the digester by a controlled iron addition to the digester feed (or recirculation) immediately upstream of the digester if possible. Caution is required because iron salts can impact the alkalinity balance in a digester.
Q: We operate drinking water plants with iron removal from shallow wells. At our wastewater treatment plant (WWTP) we use iron salts to remove phosphorus. Can the iron captured from our drinking water plants be used to remove phosphorus in our WWTP?
- Andy Warmus, Village of Algonquin
A: The effectiveness of iron removal of phosphorus in WWTPs is determined by the availability of the iron, which is influenced by mass loading (chemical dosage), dissolved iron state, competing reactions, and efficiency of contact. Theoretically, the ratio of iron to phosphorus is 1:1 (on a molar basis), but the environmental conditions in a WWTP reduce the reaction efficiency. For example, to reduce the effluent phosphorus to below 1 mg/L, an over-dosage of iron must be employed, equivalent to 3 parts iron to 1 part phosphorus (on a molar basis). The iron contained in the backwash of an iron filter will be in the form of iron oxide, not readily available for reacting with the phosphorus in the WWTP. Generally, the quantity and form of iron in filter backwash is such that it will not significantly impact the dosage of iron needed at the plant for phosphorus removal, but adding the backwash waste will help reduce iron demand.
Q: What role does struvite play in reducing phosphorus?
- Andy Warmus, Village of Algonquin
A: Struvite (magnesium ammonium phosphate) forms in municipal anaerobic digesters where the water has a magnesium hardness component. It has a tendency to form in the tanks, pumps, piping systems, and heat exchangers in contact with the anaerobic sludge, especially at valves and elbows where there is turbulence. Struvite forms as a hard, dense crystalline mass and can be a nuisance. It is typically more prevalent in plants utilizing biological phosphorus removal. Phosphorus taken up biologically in the aeration tanks is released in the anaerobic conditions of the digesters, resulting in struvite formation. Recently, there has been a growing interest in harvesting struvite from WWTPs. This interest is driven by the desire to reduce operating problems with struvite, reduce the amount of phosphorus in biosolids that are land applied, and produce a revenue stream from the struvite as a fertilizer.
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