H. Siegrist

The EAWAG Bio-P module for activated sludge model no. 3

An additional module for the prediction of enhanced biological phosphorus removal is presented on the basis of a calibrated version of ASM3. The module uses modified processes from ASM2d but neglects the fermentation of readily degradable substrate. Biomass decay is modeled in the form of endogenous respiration as in ASM3. Moreover, an additional glycogen pool and biologically induced P-precipitation were not taken into account. The module was systematically calibrated with experimental data from various batch experiments, a full-scale WWTP and a pilot plant treating Swiss municipal waste water. A standard parameter set allowed all data to be simulated.

Calibration and validation of activated sludge model No. 3 for Swiss Municipal Wastewater

Abstract ASM3 was tested against experimental data from aerobic and anoxic batches as well as full-scale experiments from various WWTPs treating Swiss municipal wastewater. A set of kinetic and stoichiometric parameters emerged from these tests. The calibrated ASM3 allows the sludge production and the denitrification capacity to be successfully modeled with a standardized set of parameters. The readily degradable inlet substrate S S,o was estimated from respiration measurements by curve fitting. This contradicts the suggestion of the IAWQ task group that S S,o in ASM3 may be approximated by the total soluble COD as determined by 0.45xa0 μ m membrane filtration. Aerobic and anoxic respiration of storage products is insignificant compared to growth on these products.

Kinetics of biologically induced phosphorus precipitation in waste-water treatment

Abstract In waste-water treatment plants with enhanced biological phosphorus removal (EBPR), a part of the phosphorus can be eliminated by chemical precipitation. In experiments with inactivated sludge containing relatively high concentrations of dissolved calcium (≈1.5xa0molxa0m −3 ) and phosphorus (≈1xa0molxa0m −3 ), a pH-sensitive and partly reversible precipitation of calcium phosphates was observed at pH values below 8.0. A dynamic model was formulated on the basis of these observations. It contains, in addition to equilibrium equations for the deprotonation(s) of phosphoric acid, carbonic acid and water, the following reactions: (1) Fully reversible precipitation of hydroxydicalcium phosphate (Ca 2 HPO 4 (OH) 2 , HDP) as an intermediate. (2) Formation of hydroxyapatite (Ca 5 (PO 4 ) 3 OH, HAP) from HDP. HAP is regarded as a kinetically stable product. A parameter estimation performed for the surface complex HDP predicts a solubility product of 10 −22.6 xa0M 5 (20°C) and a heat of dissolution (Δ H 298 ) of −88.4xa0kJxa0mol P −1 . If the solubility of HDP is exceeded, then inorganic calcium phosphate (HAP) is fixed irreversibly in the sludge with a rate of 0.5xa0mol P −3 xa0d −1 .

Prediction of recycle phosphorus loads from anaerobic digestion

Abstract In wastewater treatment plants with enhanced biological phosphorus removal, recycle phosphorus loads from sludge digestion have to be considered. Dissolved phosphorus from polyphosphate hydrolysis and degradation of organic solids is partly precipitated and adsorbed during sludge stabilization. Calcium ions and magnesium ions as well as aluminosilicates (zeolite A) from detergents are mainly involved in these processes. Experiments in a laboratory digester with pure excess sludge and mixed sludge show the extent of the fixation and indicate that recycle loads are minimized at a level of below 10%, if primary sludge is present. Even the use of prefermented primary sludge did not significantly influence this fraction. In stabilization of pure excess sludge, the dissolved phosphorus fraction comes to 20–25%. A mathematical model is presented which combines the involved processes of phosphorus release and refixation. The availability of calcium for precipitation processes is an influential parameter and may change under different wastewater conditions (hardness, zeolite content) and operation modes of the activated sludge plant (sludge age). Iron is only available for phosphorus precipitation in digestion after deducting the amount needed for the fixation of sulphur. The model allows the prediction of phosphorus recycling under different operation conditions and the comparison of process alternatives. For this purpose it is necessary to have detailed information about the fresh sludge composition with emphasis on the content of polyphosphate and inorganic phosphorus in the excess sludge.

Calibration and validation of an ASM3-based steady-state model for activated sludge systems—part I:: prediction of nitrogen removal and sludge production

The steady-state model from Siegrist and Gujer (1994) which can be used for the design and optimisation of nitrogen-removing activated sludge plants is applied to the stoichiometrics and kinetics of a validated Activated Sludge Model No. 3. It considers the wastewater composition, the effect of the electron acceptor on the average sludge production, the oxygen input into anoxic volumes, denitrification in the secondary clarifier, the temperature and various operating conditions. The organic substrate for denitrification originates from readily degradable substrate from the influent, from the hydrolysis of slowly degradable particulate substrate along the activated sludge plant and from the endogenous respiration of the biomass. The model is calibrated and validated with data from long-term full-scale and pilot-plant experiments for Swiss municipal wastewater. The most sensitive parameters as well as the uncertainty of the model prognosis for various COD-to-nitrogen ratios from inlet water and anoxic volume fractions were calculated with the aid of sensitivity analyses and Monte-Carlo simulations. Excel spreadsheets of the model for different flow schemes are available from the corresponding author.

Denitrification with methanol in tertiary filtration

Abstract The large wastewater treatment plants in Switzerland have to be extended by enhanced nitrogen removal to comply with the relevant EU directives. Denitrification in tertiary filtration is a cost-effective alternative to extended denitrification in an activated sludge system which needs additional reactor volume. Full-scale experiments in denitrification with methanol in tertiary filtration were performed at the wastewater treatment plant in Zurich-Werdholzli during a summer and a winter campaign, each lasting 4 months. One of the original 22-filter cells was equipped with a methanol dosage unit for this purpose. Denitrification rates of about 1.0 kg-N m−3 d−1 are attained at temperatures of 12–15°C. The denitrification is reduced significantly after main back-washing. Frequent back-washings (several times per day) lead to methanol breakthroughs due to biofilm loss. The yield coefficient YCOD was 0.4 kg-CODxkg-COD−1me. In spite of the methanol dosage, the quality of the filter effluent is very good during normal operation in the winter campaign. Accumulation of the nitrite intermediate product was observed in summer at temperatures of 20–22°C.

Potential of denitrification and solids removal in the rectangular clarifier

Abstract In full scale experiments, the denitrification capacity of secondary clarifiers and measures to increase it were evaluated. Due to a higher activated-sludge concentration and a lower scraper velocity, the sludge mass and thus the denitrification capacity in the secondary clarifier at the WWTP Zurich–Werdholzli was significantly increased. During the investigated period, the denitrification in the clarifier represented 37% of the total denitrification, corresponding to 19% of the total inlet nitrogen. The main part was denitrified in the inlet channel of the clarifier, so that doubling the scraper interval increased the denitrification in the clarifier by only about 14%. An approach for modelling the sludge-blanket volume was developed on the basis of measurements of the sludge-blanket height along the rectangular clarifier with transverse flow and vacuum-removal scrapers. The limiting sludge-volume load was calculated from sludge settling tests and compared with observed loads on the full-scale clarifier. Activated sludge model No. 2 was verified with the nitrate and ammonium profiles in the activated sludge tanks, return sludge and clarifier effluent.

Denitrification with methanol in tertiary filtration at wastewater treatment plant Zürich-Werdhölzli

In co-ordination with the EU-guidelines the large wastewater treatment plants in Switzerland have to be extended with enhanced nitrogen removal. Denitrification in tertiary filtration is a cost-effective alternative to extended denitrification in the activated sludge system, which needs additional reactor volume. At the wastewater treatment plant Zurich-Werdholzli full-scale experiments of denitrification with methanol in tertiary filtration were performed during a summer and a winter campaign of 4 months each. For this purpose one of the original 22 filter cells was equipped with a methanol dosage. At temperatures of 12-15°C rates of denitrification of about 1.0 kgN m −3 d −1 are attained. After main backwashing, denitrification is significantly reduced. Frequent backwashings (several times per day) led to methanol breakthroughs due to biofilm loss. The yield coefficient Y COD was 0.4 kg COD X kg −1 COD me . In spite of methanol dosage the quality of the filter effluent was very good during normal operation in the winter campaign. Accumulation of the nitrite intermediate product was observed in summer at temperatures of 20-22°C.

Calibration and validation of an ASM3-based steady-state model for activated sludge systems--part II: Prediction of phosphorus removal.

An ASM3-based steady-state model which can be used for estimating the average nitrogen-removal, sludge-production and phosphorus-removal rates of different biological phosphorus-removing systems (AAO, UCT, intermittent processes) is developed. It considers the wastewater composition, the oxygen and nitrate input in the anaerobic compartment and the interaction between biological phosphorus removal and denitrification for different operating conditions. The model is calibrated and validated with data from a number of long-term pilot and full-scale experiments for Swiss municipal wastewater. The steady-state model is adequate for a comparison of different BPR process configurations or for a first estimation of the nutrient-removal efficiency. It allows the plant performance and key parameters to be determined very quickly. Excel spreadsheets of the model for different flow schemes are available from the corresponding author.