No abstract

Two pilot-scale MBR plants were operated at the Los Angeles County Sanitation Districts' Whittier Narrows Water Reclamation Plant (WNWRP) for over a year. The plants were manufactured by Kubota Corporation and Zenon Environmental Inc. The objective of this study was to compare the long-term performance of these systems, with respect to effluent water quality, to that of the WNWRP. Water quality parameters of particular interest were COD, suspended solids, turbidity, nitrogen, priority pollutants, indicator organisms, and N-nitrosodimethylamine (NDMA). Primary effluent from the plant was used as influent to the MBRs. Average COD, total suspended solids (TSS), and ammonia concentrations in the primary effluent were 330, 100, and 24 mg/L, respectively. Key findings from the study include (1) both MBR plants achieved excellent COD (>95%), TSS (> 99%), and turbidity (typical values <0.1 NTU) removals; (2) the Zenon plant was able to achieve excellent nitrogen removal with an average effluent nitrate nitrogen concentration of 5.4 mg N/L and an ammonia nitrogen concentration typically <1 mg N/L; (3) all priority pollutants detected in the primary effluent were removed by the MBRs to levels below the detection limits or to levels equivalent to that of the WNWRP effluent; (4) both MBRs achieved >6-log removal of total coliform and fecal coliform, and >5-log removal of indigenous coliphage; (5) the median NDMA concentration of raw wastewater was 100 ng/L. This concentration was reduced by approximately 80% by the primary sedimentation and biological treatment processes of the WNWRP and Zenon MBR.

False positives for cyanide analysis in wastewaters have been reported. We examined the effects of storage time at high pH and of pH adjustments on the cyanide levels. Cyanide levels changed within the holding time allowed by Standard Methods. We also studied the difference in cyanide levels using two disinfection conditions --breakpoint chlorination and chloramination. Glycine was used as the precursor to study the cyanide formation pathways. Under breakpoint chlorination conditions, cyanide formation is complete relatively quickly and detectable cyanogen chloride is produced. On the other hand, chloramination yields cyanide through a relatively slow, base-catalyzed reaction. Chloramination followed by dechlorination with sodium arsenite and addition of NaOH results in cyanide levels that increase significantly upon reanalysis in the first 24 hours and then remain relatively constant after that time. Cyanogen chloride (CNCl) was <5 ppb in samples disinfected with chloramination. Mechanisms are proposed that explain the very different cyanide results that are obtained when disinfection is carried out under breakpoint chlorination conditions versus chloramination conditions.

Dechlorinating agents and pH adjustment are often used to preserve wastewater samples for cyanide analysis. The effects of four approved preservation protocols on the results of the total cyanide analysis of effluents from four Water Reclamation Plants (WRPs) were examined. The results differed widely, and a clear pattern emerged. Immediate analysis without pH adjustment generally gave total cyanide concentrations below the reporting limit of 5 µg/L, irrespective of the dechlorinating agents used. When the pH was adjusted to ≥ 12, a slight increase in the measured total cyanide concentration was observed when thiosulfate was used to dechlorinate the samples, and a significant increase (> 10 µg/L) was observed when arsenite was used as the dechlorinating agent. These results provide evidence that approved preservation protocols may give rise to cyanide formation in chlorinated wastewater effluent matrices.

Emission control of odors and volatile organic compounds (VOC) is a major consideration for wastewater treatment plant operation. Biotrickling filters have been employed at many wastewater treatment facilities to remove hydrogen sulfide, a major component of odor emissions. However, their effectiveness for VOC removal is relatively unknown. In this study, the Sanitation Districts of Los Angeles County (Sanitation Districts) tested a 1,500 standard cubic feet per minute (scfm) biotrickling filter system at the Joint Water Pollution Control Plant (JWPCP) in Carson, California to determine its ability of removing both odors and VOC. This biotrickling filter system included two stages; the first stage was designed for hydrogen sulfide removal and the second stage for VOC. The packing for both stages consisted of reticulated foam, and one-foot bed depth of porous rock media was later added to the first stage to minimize media plugging. The performance of the system was evaluated under several different empty bed residence times. The results indicated that the two-stage system was effective for hydrogen sulfide removal (>99%), but the VOC removal performance varied. VOC removals by use of a two-stage system will have to become more consistent for use in full-scale applications.