Edward D. Schroeder

Predicting struvite formation in digestion

Abstract The solubility constant for struvite (MgNH4PO4·6H2O) was measured in formation and dissolution equilibrium experiments. The proposed solubility constant (pKSO=13.26) accurately predicted struvite precipitation in sludge handling facilities at the Sacramento Regional Wastewater Treatment Plant, whereas the solubility constant in common use did not. An improved method was developed for predicting struvite precipitation potential in anaerobic digestion and post-digestion processes. Method improvements include consideration of ionic strength effects on ion activities, magnesium phosphate complexation effects on ion speciation, and an experimentally derived struvite solubility constant. The improved method accurately predicted struvite to be less soluble than predicted by earlier methods.

Hydrogenotrophic denitrification in a microporous membrane bioreactor

Hydrogenotrophic denitrification of nitrate contaminated groundwater in a bench-scale microporous membrane bioreactor has been investigated. To prevent microbial contamination of the effluent from the reactor the nitrate-laden water treated was separated from the denitrifying culture with a 0.02 microm pore diameter membrane. Equal pressure was maintained across the membrane and nitrate was removed by molecular diffusion through the membrane and into the denitrifying culture. The system was operated with a hydrogenotrophic denitrification culture to circumvent the addition of an organic substrate to the water. Removal efficiencies ranging from 96% to 92% were achieved at influent concentrations ranging from 20 to 40 mg/L NO3(-)-N. The flux values achieved in this study were 2.7-5.3 g NO3-N m 2d(-1). The microporous membrane served as an effective barrier for preventing microbial contamination of the product water as evidenced by the effluent heterotrophic plate count of 9 (+/- 3.5) CFU/mL. The hydrogenotrophic culture was analyzed using available 16S and 23S rRNA-targeted oligonucleotide probes. It was determined that the enrichment process selected for organisms belonging to the beta subclass of Proteobacteria. Further analysis of the hydrogenotrophic culture indicated that the organisms may belong to the beta-3 subgroup of Proteobacteria and have yet to be identified as hydrogenotrophic denitrifiers.

Denitrification and nitric oxide reduction in an aerobic toluene-treating biofilter

The presence of significant denitrification activity in an aerobic toluene-treating biofilter was demonstrated under batch and flow-through conditions. N2O concentrations of 9.2 ppmv were produced by denitrifying bacteria in the presence of 15% acetylene, in a flow-through system with a bulk gas phase O2 concentration of >17%. The carbon source for denitrification was not toluene but a byproduct or metabolite of toluene catabolism. Denitrification conditions were successfully used for the reduction of 60 ppmv nitric oxide to 15 ppmv at a flow rate of 3 L min-1 (EBRT of 3 min) in a fully aerated, 17% v/v O2 (superficially aerobic) biofilter. Higher NO removal efficiency (97%) was obtained by increasing the toluene supply to the biofilter.

Density of activated sludge solids

Abstract A technique for measuring the density of activated sludge solids is reported in this paper. Along with particle size and shape, solids density is a factor in the removal efficiency of secondary clarifiers. Density gradient centrifugation, a technique widely used in the biological sciences, was adapted to measure the density of activated sludge solids. Density gradient centrifugation consists of allowing the solid particles to settle in a fluid of continuously increasing density until the particles become stationary. At this point, particle and the fluid densities are equal. This technique was used to measure the density of activated sludge solids at several treatment plants in the Sacramento area. The density of activated sludge solids at a given treatment plant over a period of time was relatively constant. However, at different treatment plants densities ranged from a low of 1.02 to a high of 1.06 g/ml, indicating that plant operations may influence solids density. Using the measured density of secondary solids, variation in settling velocities was estimated using the following assumptions, (1) discrete particle settling; (2) an average particle diameter of 100 μm; and (3) laminar flow conditions. The corresponding settling velocities for particles with densities of 1.02 and 1.06 g/ml are 16.9–26.8 m/day.

Trends in application of gas-phase bioreactors

Gas-phase biological processes areeffective systems for treating air contaminatedwith biodegradable compounds. The greatestamount of experience with gas-phase processesexists with odor control and sulfide oxidation.Organic contaminant concentrations are limitedby oxygen transfer and clogging to values ofapproximately 1000 ppmv and loading rates ofapproximately 20 g/m3·h with empty bed contacttimes and air flux values of approximatelyone-minute and 1 m3/m2·min, respectively.Outlet contaminant concentrations attainableare generally near the non-detect level andnearly always below 50 ppbv. Very fewcontaminant concentration profiles have beenpublished and most performance and processresponse modeling has been based on in/outcontaminant values. Based on limited pilot andlaboratory scale profile data, it appears thatmany systems treating less soluble organics maybe fully or partially mass transport limited.The principal control problem in biofilters ismoisture content. Systems should be designedwith capacity to periodically add moisture tothe bed. In systems treating sulfides orchlorinated organics acid is produced and pHcontrol and corrosion can be problems. Cloggingdue to excess microbial growth results inincreased head loss, channelization, anddecreased performance. Initial head loss isusually less than 10 mm/m but increases as runtimes become extended. Typical of plug flowbiological processes, the highest removal ratesoccur near the inlet. Transient contaminantconcentration pulses tend to result in deeperpenetration into the bed where activity islower.

Relationship between the observed cell yield coefficient and mean cell residence time in the completely mixed activated sludge process

Abstract A laboratory scale activated sludge unit was operated under highly controlled conditions. Hydraulic detention time was maintained constant and between 6 and 6.5 h. Sludge wasting rate was the only process parameter varied. By varying sludge wasting rate the mean cell residence was varied between 2 and 18 days. Within the span of 2–18 days only a minimal change in effluent COD was observed. Sludge production, however, was significantly altered. The data obtained were found to be described by both linear equation and an exponential equation.

Formation of density currents in secondary clarifier

Abstract Density currents exist in most activated sludge secondary clarifiers, but the causes are controversial. The purpose of this study was investigation of the internal flow regimes to identify and evaluate the principal causes of density currents in secondary clarifiers. This study was part of a project focused on controlling the impact of density currents on the performance of activated sludge secondary clarifiers. A 1:25 scale model of a conceptualized prototype rectangular clarifier was used and was operated by the Froude model law. Experiments were conducted in the laboratory with tap water and in the field with activated sludge mixed liquor. Two inlet baffle, intermediate baffle and effluent weir configurations and three sludge draw-off configurations were used in the experiments. In all cases, one-half of the influent flow was removed through the sludge draw-off ports located in the tank bottom. In each experiment, velocity pattern observations and temperature measurements were made throughout the tank. Observed flow patterns were very similar in the laboratory experiments with tap water and the field experiments with mixed liquor. Temperature-driven density currents formed quickly under all inlet baffle and outlet configurations and were extremely stable. Depth of the density currents was inversely related to the difference between temperatures of the influent and tank contents. A temperature difference as small as 0.2°C resulted in formation of density currents. Moreover, the formation of a density current in the tank was independent of the amount of suspended solids in the entering suspension. Temperature appears to be the primary cause of density currents in secondary clarifiers and the type of density current, surface or bottom, will depend on whether the influent is warmer or cooler than the tank contents. In the field, due to the difference between the air and influent mixed liquor temperature, the formation of density currents in secondary clarifiers appears to be inevitable and little affected by the presence or type of inlet or intermediate baffles.

A two-state microbial growth kinetics model

Abstract This study considers the possibility that dispersed bacteria and bacteria aggregated in floc possess different physiological states and that aggregation of dispersed cells to form floc particles is a reversible rate process. A suitable kinetic model of flocculation is proposed, in which the floc phase and dispersed phase bacteria exhibit different growth rates. Transient responses to changes in dilution rate and inlet concentration are investigated. Results are compared with experimental observations. Variation in observed yield factor and hysteresis of specific growth rate are observed.

Importance of the BOD plateau

ThE BIOChEMICAL oxygen demand test has been a subject of continual discussion and controversy. MONTGOMERY (1967) has pointed out that the standard test is slow, that there is little relation between laboratory and field conditions, and that the test possesses no theoretical significance and lacks reproducibility. Yet the five day 20°C BOD is the standard measure of the organic pollution potential of a waste water. All other methods of measurement of oxygen demand have been compared or correlated with the five day BOD. Thus the use of the Warburg Respirometer to measure BOD was only tentatively accepted by Standard Methods in the l lth edition and was eliminated from the 12th edition (1966). Biochemical oxygen demand data have been utilized for two purposes: to predict the total oxygen demand of a waste water and to predict the rate of deoxygenation. SWILLEY et al. (1964), I~TD/HRBERGER et aL (1964) and GULEVICH et al. (1968) have demonstrated the importance of diffusion on the rate of BOD exertion. Their work indicates that rate data obtained from a laboratory experiment, using quiescent bottles, would not be applicable to streams or waste treatment processes. The only use for BOD rate data would then be in predicting the total oxygen demand. Prediction of the total oxygen demand from five day BOD values is a questionable procedure. An assumption must be made with respect to the mathematical description of the BOD progression with time. A first order model is normally used [(dc/dt) = -kc]. Only two data points are needed to determine the rate constant, k, for which the use of a five day test would seem unreasonable. Any point on the curve should provide the necessary information. PhELPS (1944) states that the adoption of the five day test as the standard was based upon the unreliability of data taken previous to five days. The unreliability was assumed to be due to variation in the lag time before BOD exertion began. This fact contradicts the assumed first order models and is particularly important because both the model and experience indicate that the highest oxygen uptake rates occur early in the test. The model is therefore unreliable for the most sensitive part of the BOD progression and is usable only in the least sensitive region.

A method for measuring haloform formation during wastewater chlorination

Abstract An experimental methodology was developed to measure haloform formation in chlorinated wastewater samples using a flexible bag, zero-head space, reactor. A flexible reactor vessel is advantageous when the reaction products are volatile since successive samples can be collected over time without creating a head-space and thereby eliminating partitioning of volatile reaction products from the aqueous phase to the gaseous phase. Grab samples of three kinds of wastewaters; raw wastewater, nitrified secondary effluent, and partially nitrified secondary effluent, were chlorinated at two levels of chlorine doses (Cl 2 ). The magnitude and rate of total trihalomethane (TTHM) production was estimated based on the formation of chloroform (CHCl 3 ), bromodichloromethane (CHCl 2 Br), dibromochloromethane (CHClBr 2 ) and bromoform (CHBr 3 ).