Environ. Sci. Technol. 2006, 40, 202-207
Dimethyl Sulfoxide (DMSO) Waste Residues and Municipal Waste Water Odor by Dimethyl Sulfide (DMS): the North-East WPCP Plant of Philadelphia DIETMAR GLINDEMANN,* JOHN NOVAK,† AND JAY WITHERSPOON‡ Glindemann Environmental Services, Goettinger Bogen 15, D-06126 Halle, Germany
This study shows for the first time that overlooked mg/L concentrations of industrial dimethyl sulfoxide (DMSO) waste residues in sewage can cause “rotten cabbage” odor problems by dimethyl sulfide (DMS) in conventional municipal wastewater treatment. In laboratory studies, incubation of activated sludge with 1-10 mg/L DMSO in bottles produced dimethyl sulfide (DMS) at concentrations that exceeded the odor threshold by approximately 4 orders of magnitude in the headspace gas. Aeration at a rate of 6 m3 air/m3 sludge resulted in emission of the DMS into the exhaust air in a manner analogous to that of an activated sludge aeration tank. A field study at the NEWPCP sewage treatment plant in Philadelphia found DMSO levels intermittently peaking as high as 2400 mg/L in sewage near an industrial discharger. After 3 h, the DMSO concentration in the influent to the aeration tank rose from a baseline level of less than 0.01 mg/L to a level of 5.6 mg/L and the DMS concentration in the mixed liquor rose from less than 0.01 to 0.2 mg/L. Finding this link between the intermittent occurrence of DMSO residues in influent of the treatment plant and the odorant DMS in the aeration tank was the key to understanding and eliminating the intermittent “canned corn” or “rotten cabbage” odor emissions from the aeration tank that had randomly plagued this plant and its city neighborhood for two decades. Sewage authorities should consider having wastewater samples analyzed for DMSO and DMS to check for this possible odor problem and to determine whether DMSO emission thresholds should be established to limit odor generation at sewage treatment plants.
Introduction Modern municipal wastewater systems have learned to mitigate the classic “rancid” odor of fatty acids or the “rotten egg” odor of hydrogensulfide (H2S) by the use of air filters and proper aeration tank system design and operation. One odor that continues to resist these control strategies is often described as “rotten cabbage” or “canned corn” like. This odor can be attributed to the compound dimethyl sulfide (DMS, CAS 75-18-3), which has a very low odor detection threshold in air. DMS is a clear evaporable liquid with a boiling * Corresponding author e-mail:
[email protected]. † Virginia Tech, Blacksburg, VA. ‡ CH2M Hill, Oakland, CA. 202
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 40, NO. 1, 2006
point of 38 °C. DMS is slightly soluble in water. The Henry law constant of DMS is about 170 Pa m3/mole at 25 °C. The water/gas partition coefficient of DMS is about 15. DMS can be produced naturally by the reduction of dimethyl sulfoxide (DMSO, CAS 67-68-5) in the global sulfur cycle (1-3) where DMSO is a byproduct of algal growth. Many microorganisms (including those microbial communities found in sewage and activated sludge) can reduce DMSO to DMS under anoxic conditions similar to those for nitrate reduction (4-8). The redox potential for the reduction of DMSO at pH 7 of 160 mV (9) is lower than for nitrate reduction, but higher than for sulfate reduction. Studies of the microbial reduction of DMSO (7, 8) used very high concentrations of DMSO (up to 30 mass %), as a method to measure biological activity, but did not investigate the odor potential of low mg/L additions of DMSO as might occur in sewage. The conversion of DMSO to DMS by activated sludge (VSS) was in the order of 1-3 µmol g-1 h-1 (7). DMSO is an excellent industrial solvent that does not evaporate from water and has almost no odor. DMSO has no significant health effects (an OSHA PEL limit has not been established). DMSO is an important industrial chemical (10). The annual production of DMSO is about 50 000 metric tons. Consumers use DMSO in household chemicals and pharmaceuticals; industries use DMSO as a solvent in numerous process and product applications and typically recycle the chemical. Concentrated DMSO wastewater (1000 mg/L DMSO) has been treated (11, 12) by small industrial WWTP units dedicated to the elimination of DMSO. Residues of DMSO recycling are believed to be disposed into sewers and the general environment. Industries that generate these DMSO emissions are not subject to the reporting requirements of Section 313 of Title III of the Superfund Amendments and Reauthorization Act (SARA) of 1986 and 40 CFR Part 372. No “threshold” quantity of DMSO emission is defined to guide DMSO users whether they have to report any DMSO discharge to the EPA or to communicate discharges to wastewater authorities. There are no reports in the literature of actual environmental odor problems at municipal wastewater plants resulting from DMSO discharges nor are there protocols available to prepare authorities to explicitly deal with municipal wastewater odors from DMSO. Current treatment protocols would only take notice of DMSO if it were to increase the BOD of wastewater and thereby impact the treatment capacity. The main goal of this investigation was to demonstrate that disposal of waste DMSO by industries is likely a cause of environmental odor in sewers, municipal wastewater treatment plants, and in the general aquatic environment. In particular, the municipal North-East Water Pollution Control Plant (NEWPCP) of the Philadelphia Water Department (PWD) and the community residing along one fence line had been plagued with the problem of sporadic “canned corn” odors by volatile organic sulfur (VOS) since the 1980s. The problem was persistent despite the installation of efficient aeration technology during that period, and the costly addition of a chemical oxidant since early 2003. It proved to be a challenging riddle that it was a well-aerated activated sludge system that produced the most VOS sporadically with no link to actual WWTP plant operation conditions. An initial investigation in 2003 indicated that more than 80% of VOS was dimethyl sulfide (DMS) when VOS concentration was higher than background value, DMS production was strongly related with a specific incoming wastewater 10.1021/es051312a CCC: $33.50
2006 American Chemical Society Published on Web 11/25/2005
stream of the plan,t and activated sludge is a powerful factor of DMS production (13). A research proposal submitted by the first author to PWD for DMSO source control was the key to solving the persistent odor problem of the NEWPCP plant. It was proposed that a hitherto undetected industrial waste stream of DMSO was being discharged to the sewer, and bacteria in the sewer and treatment plant transformed DMSO into DMS, causing the intermittent odor problem. In this study, we report the results of a field study conducted at the NEWPCP municipal WWTP by the Philadelphia Water Department and of a supplementary benchscale study to show that relatively small industrial DMSO waste residues can cause environmental odor problems in municipal wastewater plants.
Materials and Methods Field Study. Objective and Description of the Field Study. The field study at the NEWPCP plant of the PWD was a DMSO source control program to identify suspected industrial sources of DMSO waste streams, and monitor and evaluate the significance of sporadic DMSO discharges as a cause of the “canned corn” odor problems resulting from the formation of DMS. Results from the field investigation were provided by PWD. Description of the NEWPCP Municipal Wastewater Plant. PWD operates Pennsylvania’s second largest (0.8 million m3/day or 210 mgd) secondary wastewater treatment facility (NEWPCP) in the Bridesburg section of Philadelphia. The plant is fed from four sewers, one of which is the Delaware low level sewer (DLL) that supplies approximately 55% of the total flow. Along with numerous other permitted industrial users discharging into NEWPCP, a major chemical plant discharges several miles upstream from the plant into the DLL sewer. The influent is split, after preliminary treatment, between two sets of primary sedimentation tanks (PST1 and PST2). The two PST feeds flow into nonaerated activated sludge selector zones of seven 2-step-feed aeration basins. Mixing of the sewer flows is incomplete, so that the DLL sewage (which is rich in industrial wastes) flows mainly into PST1. Two sets of secondary clarifiers provide final settling. The membrane-aeration system used at the plant is adequate for suppressing H2S and mercaptan odors, but a distinct “canned corn” odor is occasionally emitted from the aeration tanks. A chemical oxidant is pumped continuously into the activated sludge system at a concentration level of approximately 10 mg/L in sludge to mitigate the intermittent odor typical of the aeration tank. During odor events, the dosage of oxidant is increased. The wastewater had been routinely analyzed for a list of 80 volatile organic (VOC) compounds. Although the canned corn odor problem is said to have been occurring off and on since the 1980s, DMSO data were not collected before the field study started. The industrial facility determined to be the source of this discharge is a customer of PWD’s NEWPCP plant and has used DMSO in their industrial process for about two decades. DMSO is used for a variety of manufacturing operations that do not fluctuate seasonally. DMSO is recycled for reuse. However, the recycling process is not complete and leaves DMSO waste residues that are periodically washed into the DLL sewer. The period of the sewer wash cycle is approximately 40 h. However, odor problems do not occur cyclically every 40 h. It is likely that washes from different cycles could overlap and cause unpredictable spikes of DMSO in the sewer. Water Sampling. The data in this report are from samples collected on April 21, 2004. This day is representative for the other sampling days. Water samples were taken regularly at half-hour intervals throughout the day during a scheduled DMSO discharge. Wastewater samples were collected at the
sewer near the influent to the Set 1 Primary Tanks, in the effluent from Set 1 Primary Tanks, in the aeration tanks, and in the return activated sludge (RAS) taken from the pipe before it entered the aeration tank. Sample Preparation and Analysis. The samples were analyzed by gas chromatography and compared with standards of the analyzed sulfur compounds. No further description of materials used, sample preparation, or analytical methods was provided by PWD. Bench-Scale Experiments. Objective and Description of the Bench-Scale Experiments. Bench-scale experiments in the laboratory were designed to determine if low (1-10 mg/L) concentrations of DMSO in wastewater sludge would produce significant quantities of DMS. The experiments involved the addition of DMSO to closed bottles containing treatment plant sludge from various location in the plant and analysis of the headspace gases. Materials for Bench-Scale Experiments. The chemicals DMSO, Na2S, Na-mercaptanoate, and HCl were of analytical grade (Merck, Darmstadt, Germany). Per-deuterated DMSO (d6) and perdeuterated DMS (d6) were purchased from SigmaAldrich. Chemical color test tubes type dimethyl sulfide 1a (Draeger Mercaptan 0.5/a, Draeger H2S 0.5a) were purchased from the Draeger Company, Luebeck. The batch-incubation containers were PET plastic beverage bottles obtained from a grocery store. These bottles have been tested and were found to be sufficiently airtight and chemically inert (13). Description of Sludge and Water Samples Used for the Bench-Scale Experiments a-d. A variety of sludges were used to study the conversion of DMSO to DMS. These samples were collected from different locations in the treatment plant in order to determine the likely locations for DMS generation. On the day of these tests, the plant did not have any measurable DMSO inflow and the plant did not have DMSOdischarging industrial customers. The microscopically determined microbial composition of the samples was typical of municipal wastewater treatment. The following sludge samples were used. Return activated sludge (RAS, volatile suspended solids [VSS] content of 4.2 g/L suspended in supernatant from the return sludge flow) was used for experiments a, b, and c. A primary sludge-influent mixture (VSS ) 0.3 g/L), activated sludge (VSS ) 0.8 g/L), and digested stored sludge cake (VSS ) 215 g/kg), were used for experiment b. The digested stored sludge cake was a dewatered cake from anaerobically digested sludge (digester SRT ) 30 days), made free of mercaptan and DMS by digestion for an additional 20-day period in a bottle prior to the experiment. A sample of river water (VSS ) 0.1 g/L) from the German Saale river near the town of Halle was used for experiment d. Method of Anoxic Incubation in Bottles. The liquid sample to be tested (see experiments a, b, c, and d below) and DMSO were poured into incubation bottles through a hole in the plastic cap to allow intermediate gas sampling and re-sealing with clear tape. The sample was then purged with 20 L of nitrogen per liter of sludge to remove any initial volatile sulfur compounds and oxygen. Air (21% oxygen) was then added to the nitrogen headspace to set the initial oxygen concentration at 5%. Only 1% oxygen was added to the plant influent wastewater and river water. The bottles were then incubated for up to 72 h standing upright at room temperature (22 °C) and were periodically gently shaken so that biomass could mix with water. The oxygen consumption in the bottles was checked with a dissolved-oxygen electrode for qualitative purposes until oxygen was not detected (
