M. Robin Collins

Microbial populations, activities and carbon metabolism in slow sand filters

The bacterial population distributions and activities in municipal slow sand filters from three separate facilities using different management practices were evaluated to better understand factors influencing bacterial population distributions and metabolism of dissolved aquatic organic matter (AOM). Source waters and filter effluents were evaluated for direct and viable bacterial counts as well as AOM apparent molecular weight (AMW) distributions. Filter media were evaluated for direct and viable counts, biomass (as protein), extractable Fe and Mn, tetrazolium dye respiratory activity and [14C]benzoate mineralization. For a given facility, population distributions were examined from cores within a filter, from cores between different filters and from cores from the same filter during different seasons. There was remarkably little difference in distributions for a given facility. There were marked differences between facilities. Two of the plants that practice conventional cleaning techniques (Springfield, Mass. and New Haven, Conn.) showed typically high populations and biomass in the schmutzdecke and sharply declining populations and biomass with depth. Strong intercorrelations between acriflavine direct cell counts (AFDC), specific plate counts, biomass as Folin-reactive material (FRM) and extractable Fe and Mn were seen with depth in filter cores. The plant that practices wet harrowing as a cleaning technique (West Hartford, Conn.) showed schmutzdecke-like high populations and biomass down to the depth of harrowing (30 cm). The reduction of tetrazolium chloride as a measure of respiratory electron transport and the ability to mineralize [14C]benzoate was significantly greater (P < 0.05) in West Hartford samples; up to 14% of the schmutzdecke direct counts reduced tetrazolium and mineralization rates up to 35 ng/ml/h (rate constants of 0.65 d−1) were observed. Removal of the <0.5 k AMW fraction of the AOM was particularly pronounced at the West Hartford facility, corroborating the observed metabolism of benzoate and suggesting that smaller molecular weight AOM is supporting growth in the filters. Harrowing therefore maintains high levels of acclimated and metabolically active biomass in the slow sand filters.

Toward optimum control of the activated sludge process with reliability analysis

Abstract A scheme to maximize reliability and to identify upsets caused by influent and process disturbances of the activated sludge treatment system with recycle is presented. Reliability is the probability that the effluent wastewater quality meets the maximum contamination standard for BOD5. The plant is assumed to operate under steady-state conditions. Since influent flow and substrate concentration have significant variation, they are introduced into the model as random variables. thus concerns about plant stability are addressed. Reliability is expressed as a function of return sludge concentration and recycle flow. Control charts are used to identify upsets caused by influent disturbance, combinations of flow and BOD5. and process disturbance, combinations of F/M (food to microorganism ratio) and MLSS (mixed liquor suspended solids). Recommendations to maximize reliability in the overall context of plant performance are presented and discussed.

Predicting THM concentration in treated water with highly correlated data

Chlorine is an effective disinfectant in killing bacteria and viruses; however, trihalomethanes (THM) and several chlorination by-products of the drinking water treatment process are suspected carcinogens. In order to develop treatment strategies to minimize THM production, mathematical models containing appropriate raw water quality and control predictor variables are needed. Some of these predictor variables are known to be highly correlated; therefore, when collectively introduced into a regression model, questions about the validity of the model arise. Condition indexes and variance decompositions, as well as ridge regression, are used to identify the degree and causes of ill-conditioning. Path analysis is used to support the inclusion of predictor variables in the final model.

Life cycle environmental and economic implications of small drinking water system upgrades to reduce disinfection byproducts

Many of the small drinking water systems in the US that utilize simple filtration and chlorine disinfection or chlorine disinfection alone are facing disinfection byproduct (DBP) noncompliance issues, which need immediate upgrades. In this study, four potential upgrade scenarios, namely the GAC, ozone, UV30, and UV186 scenarios, were designed for a typical small drinking water systems and compared in terms of embodied energy, carbon footprint, and life cycle cost. These scenarios are designed to either reduce the amount of DBP precursors using granular activated carbon filtration (the GAC scenario) or ozonation (the ozone scenario), or replace the chlorine disinfection with the UV disinfection at different intensities followed by chloramination (the UV30 and UV186 scenarios). The UV30 scenario was found to have the lowest embodied energy (417 GJ/year) and life cycle cost (

Removing natural organic matter by conventional slow sand filtration

0.25 million US dollars), while the GAC scenario has the lowest carbon footprint (21 Mg CO2e/year). The UV186 scenario consistently presents the highest environmental and economic impacts. The major contributors of the economic and environmental impacts of individual scenarios also differ. Energy and/or material consumptions during the operation phase dominate the environmental impacts of the four scenarios, while the infrastructure investments have a noticeable contribution to the economic costs. The results are sensitive to changes in water quality. An increase of raw water quality, i.e., an increase in organic precursor content, could potentially result in the ozone scenario being the least energy intensive scenario, while a decrease of water quality could greatly reduce the overall competitiveness of the GAC scenario.