Feasibility of focused-pulsed treated waste activated sludge as a supplemental electron donor for denitrification.

We evaluated the feasibility of using waste activated sludge (WAS) from a wastewater treatment plant as an internal electron donor to fuel denitrification, by increasing its bioavailability with Focused-Pulsed (FP) technology. The focused-pulsed treatment of WAS (producing FP-WAS) increased the semi-soluble chemical oxygen demand (SSCOD) by 26 times compared with the control WAS. The maximum denitrification rate of FP-WAS (0.25 g nitrate-nitrogen [NO3- -N]/g volatile suspended solids [VSS] x d) was greater than for untreated WAS (0.05 g NO3- -N/g VSS x d) and methanol (0.15 NO3- -N/g VSS x d). Centrifuging out the larger suspended solids created FP-centrate, which had a rate (0.14 g NO3- -N/g VSS x d) comparable with that of methanol. Thus, FP treatment of WAS created SSCOD, which was an internal electron donor that was able to drive denitrification at a rate similar to or greater than methanol. One trade-off of using FP-WAS for denitrification is an increase in total Kjeldahl nitrogen (TKN) loading. While FP-WAS achieved the lowest total nitrogen and NO3- -N concentrations in the batch denitrification test, its final ammonia-nitrogen (NH3-N) concentration was the highest, as a result of the release of organic nitrogen from the FP-treated biomass; FP-centrate had less release of total soluble nitrogen. While the return of total nitrogen (TN) is small compared with the SSCOD, the effects of the added nitrogen loading need to be considered.

Comparison between acetate and hydrogen as electron donors and implications for the reductive dehalogenation of PCE and TCE.

Bioremediation by reductive dehalogenation of groundwater contaminated with tetrachloroethene (PCE) or trichloroethene (TCE) is generally carried out through the addition of a fermentable electron donor such as lactate, benzoate, carbohydrates or vegetable oil. These fermentable donors are converted by fermenting organisms into acetate and hydrogen, either of which might be used by dehalogenating microorganisms. Comparisons were made between H2 and acetate on the rate and extent of reductive dehalogenation of PCE. PCE dehalogenation with H2 alone was complete to ethene, but with acetate alone it generally proceeded only about half as fast and only to cis-1,2-dichloroethene (cDCE). Additionally, acetate was not used as an electron donor in the presence of H2. These findings suggest the fermentable electron donor requirement for PCE dehalogenation to ethene can be reduced up to 50% by separating PCE dehalogenation into two stages, the first of which uses acetate for the conversion of PCE to cDCE, and the second uses H2 for the conversion of cDCE to ethene. This can be implemented with a recycle system in which the fermentable substrate is added down-gradient, where the hydrogen being produced by fermentation effects cDCE conversion into ethene. The acetate produced is recycled up-gradient to achieve PCE conversion into cDCE. With the lower electron donor usage required, potential problems of aquifer clogging, excess methane production, and high groundwater chemical oxygen demand (COD) can be greatly reduced.

Simulated and experimental evaluation of factors affecting the rate and extent of reductive dehalogenation of chloroethenes with glucose.

Carbohydrates such as molasses are being added to aquifers to serve as electron donors for reductive dehalogenation of chloroethenes. Glucose, as a model carbohydrate, was studied to better understand the processes involved and to evaluate the effectiveness for dehalogenation of different approaches for carbohydrate addition. A simulation model was developed and calibrated with experimental data for the reductive dehalogenation of tetrachloroethene to ethene via cis-1,2-dichloroethene. The model included fermentors that convert the primary donor (glucose) into butyrate, acetate and hydrogen, methanogens, and two separate dehalogenator groups. The dehalogenation groups use the hydrogen intermediate as an electron donor and the different haloethenes as electron acceptors through competitive inhibition. Model simulations suggest first that the initial relative population size of dehalogenators and H(2)-utilizing methanogens greatly affects the degree of dehalogenation achieved. Second, the growth and decay of biomass from soluble carbohydrate plays a significant role in reductive dehalogenation. Finally, the carbohydrate delivery strategies used (periodic versus batch addition and the time interval between periodic addition) greatly affect the degree of dehalogenation that can be obtained with a given amount of added carbohydrate.

Effect of low solids retention time and focused pulsed pre-treatment on anaerobic digestion of waste activated sludge.

The interacting effects of Focused Pulsed (FP) treatment and solids retention time (SRT) were evaluated in laboratory-scale digesters operated at SRTs of 2-20 days. Anaerobic digestion and methanogenesis of waste activated sludge (WAS) were stable for SRT ≥ 5 days, but the effluent soluble organic compounds increased significantly for SRT=2 days due to a combination of faster hydrolysis kinetics and washout of methanogens. FP treatment increased the CH(4) production rate and TCOD removal efficiency by up to 33% and 18%, respectively, at a SRT of 20 days. These effects were the result of an increase in the hydrolysis rate, since the concentrations of soluble components remained low for SRT ≥ 5 days. Alternately, FP pre-treatment of WAS allowed the same conversion of TCOD to CH(4) with a smaller SRT and digester size: e.g., 40% size savings with a CH(4) conversion of 0.23 g CH(4)-COD/g COD(in).

Effects of solids retention time on methanogenesis in anaerobic digestion of thickened mixed sludge.

When a bench-scale digester fed thickened mixed sludge was operated over an SRT range of 4-20 days, removal efficiencies for total chemical oxygen demand and volatile suspended solids declined with decreasing SRT (especially <10 days), but methanogenesis was stable for SRT as low as 5 days. Quantitative PCR analyses showed that methanogens declined steadily for SRT<10 days, with the acetate-cleaving Methanosaetaceae becoming more dominant. Clone-library analyses indicated significant shifts in bacterial population from 20 to 4 day SRT: declining Chloroflexi (28 to 4.5%) and Syntrophomonas (9 to 0%), but increasing Bacteroidetes (12.5 to 20%) and two acetogenic genera belonging to the phyla Firmicutes and Spirochaetales (6.3 to 12%). Thus, the decrease in the apparent hydrolysis constant (khyd-app) with higher SRT and the process limiting size of Methanosaetaceae with the lower SRT are proactive signs for defining rate limitation in anaerobic digestion.