Chapter 2: Manuscript 1 Inter-relation between trivalent cations and labile proteins and its influence on odor production from wastewater sludges and biosolids.
Sivarangan Subramanian+, John T. Novak+, and Sudhir Murthy* Virginia Polytechnic Institute and State University, Department of Civil and Environmental Engineering, Blacksburg, VA.* DC Water and Sewer Authority, Washington D.C.
(Planned for submission to Water Environment Research)
Odor production from solids handling processes in wastewater treatment has long been a matter of concern. In this study, the production of malodorous odors from lime stabilized biosolids at three wastewater treatment plants in the Washington DC area was investigated along with the role of trivalent cations in odor production.
In phase one, it was found that production of total sulfur and TMA odors over time was influenced by the lime dose and the incorporation of lime into sludge. Shear conditions present in belt filter press produced higher quantities of odors compared to a vacuum filter. Pre liming, used to provide better lime incorporation into sludge, was found to be effective in one case and ineffective in another. Increased biological activity was thought to be the reason for change in odor patterns, with an increase in sulfur odors under poorly limed conditions and a decrease in TMA odors as the pH goes down and TMA becomes less volatile.
In phase two, the inter-relation between trivalent cations, labile proteins and odor potential was investigated. It was determined that odor potential is directly related to the labile protein content in sludge/biosolids. Furthermore, as the Al/Fe ratio increased, the labile proteins was observed to decrease. This indicates that trivalent cations have an important role in binding the proteins thus controlling their bioavailability and hence effecting odor production. This was found true for most sludge irrespective of their liming status and independent of upstream process conditions.
sludge, total sulfur odors, trimethylamine, trivalent cations, odor potential, labile proteins, shear
More than ten million tons of biosolids are generated in the United States each year, of which about 65% is recycled beneficially. One of the barriers to the beneficial reuse of biosolids is odor generation. Opposition to land application can be reduced if odors can be minimized.
Wastewater sludges/biosolids are abundant sources of food for microorganisms in the form of proteins, amino acids and carbohydrates (EPA, 2000a). The degradation of these compounds is considered to be the cause of malodorous odor emissions. The odors typically identified from a wastewater processing facility or land application site containing biosolids are reduced sulfur compounds, nitrogen based compounds and organic fatty acids (WEF, 1995). Stored sewage sludges continuously release malodorous odors and tend to smell worse during transport and ensuing movement due to the sudden release of accumulated odors (Lambert and McGrath, 2000).
Various processes are used for stabilizing sludge which include anaerobic and aerobic digestion, lime stabilization and composting. Sulfur based odors are obtained from dewatered sludge both pre and post digestion. Sulfur compounds typically detected in wastewater sludges include hydrogen sulfide (HS), methane thiol (MT), dimethylsulfide (DMS) and dimethyldisulfide (DMDS). Anaerobically digested sludges are known to emit volatile sulfur compounds (VSCs) comprising mainly of MT (Novak et.al, 2004).
Trimethylamine (TMA) odors are typically associated with lime stabilized sludges where the pH is 12 or greater. TMA has a pKa of 9.81 (Novak et.al, 2002) and has a characteristic fishy odor. TMA odors are known to emanate from lime stabilized biosolids along with DMDS (Kim et.al, 2001). TMA formation is due to microbial breakdown of organic compounds coupled with pH and temperature mediated physico-chemical processes (Murthy et.al, year, 2002c). As the pH is raised above the pKa for TMA, liquid phase TMA is converted to its unionized and volatile form followed by gradual emission over time.
Proteins, containing an amino acid group as monomeric building blocks, are considered to be the primary precursors for formation of odor compounds. Cysteine and methionine are known to be present in proteins extractable from activated and anaerobically digested sludges. This protein remains unmetabolized as long as the digestion/stabilization process does not make them bioavailable. The bioavailability of proteins is considered to be the key, but poorly understood, process in the release of odor compounds from wastewater sludges/biosolids. The cation bridging model postulated by Higgins et.al (1997) explains the role of cations acting as a bridge connecting the negatively charged sites of the biopolymer within sludge. Proteins were found to be major constituents in activated sludge and they concluded that proteins had a more important role in defining floc structure because of their predominance in electrostatic bond formation with multivalent cations.
Lomans et.al (1999) showed that methanogenesis is the major mechanism for DMS and MT consumption in fresh water sediments. In another of their experiments, methlylation reactions were found to transform sulfide and MT into MT and DMS respectively. They further demonstrated that methanogens and sulfate reducing bacteria in the presence of sulfate can degrade MT and DMS, thus continuing the VSC degradation cycle (Lomans et.al, 2002a). These findings were corroborated by Higgins and co workers (2002).
Shearing of sludge that occurs in dewatering devices like centrifuge and belt filter press is known to significantly influence the production of odors. Basu et.al (2004) reported that shear increases the digestibility of sludge, leading to more volatile solids (VS) destruction. This is due to exocellular biopolymers being rendered bioavailable under application of shear. Muller et.al (2004) further showed that the proteins in exocellular biopolymer would degrade following shear, leading to odors.
Studies conducted by Murthy et.al (WEFTEC 2002a) revealed that a high solids centrifuge gave a higher production of VSCs compared to a low solids centrifuge. A combination of screw conveyance following dewatering by centrifugation produced almost three times more odor than that produced by centrifugation alone. These workers also developed a model correlating the increase in labile protein content in biosolids to the increase in applied shear. The subsequent degradation of labile proteins upon being subjected to shear resulted in an increased production of VSCs, particularly MT (an order of magnitude of almost 16 times) compared to unsheared conditions.
Polymer added to sludge to aid the dewatering process is documented to contribute towards increasing odor potential (Higgins et.al, 2002; Muller et.al, 2004). This was reported to be due to association of proteins from floc with added polymer. Murthy et.al (2001, 2002b) have theorized that cationic polymer conditioning of sludge prior to dewatering raises odor potential, especially for production of TMA. Proteins and polymers in biosolids are thought to be enyzmatically broken down upon consequent lime addition, cause TMA and DMDS to be released (Kim et.al, weftec 2001; Turkmen et.al, 2004.
Trivalent cations (iron and aluminum) are thought to have a major impact on sludge characteristics due to their strong association with sludge matrix (Park et.al, 2004). Iron is known to undergo reduction from Fe (III) to Fe (II) by iron reducing bacteria (Rasmussen et.al,1996; Caccova et.al, 1996). Ferric iron has been shown to have a higher affinity for proteins (Novak et.al, 2003). They also concluded that large quantities of proteins are released during iron reduction during anaerobic digestion process which can be attributed to specific binding of proteins with ferric iron.
The objectives of this study were
- To determine the odor profile from limed biosolids
- To determine effect of lime incorporation in dewatered sludges
- To determine the inter-relations between trivalent cations, labile proteins and odor potential from wastewater sludges/biosolids.
MATERIALS AND METHODS
This study was conducted in two phases. The first phase was an investigation of the odor production from three wastewater treatment plants in the Washington DC area that use lime stabilization for odor control. Wastewater sludge samples were obtained from the Blue Plains, Parkway and Piscataway treatment plants. Samples included both limed as well as unlimed from the three plants. Phase II was comprised of experiments conducted at Virginia Tech to study the interaction between trivalent metals in sludge, labile proteins and odor potential.
Sample preparation and analysis
Sludge/biosolids samples were shipped directly from the three plants in 260 mL PET bottles on ice through overnight shipping. Two sets of samples were studied. Short term samples were analyzed when received for TMA and sulfur compounds in headspace following which, the bottles were opened for withdrawal of samples for analyses of metals and proteins. The long term samples were monitored for odorous gases in the headspace and analyzed on days 1, 2, 3,5, 8, 14, 28 and 56 for the analytes of interest. Samples for headspace analysis were incubated at 25 degree C for the duration of experiment. All other samples were stored at 4 degree C for preservation until the start of the experiment.
Wastewater sludges were obtained from two local wastewater treatment plants, Blacksburg, VA and Radford, VA. Samples were collected within 24 hours of start of the experiment, and were stored at 4 degree C for preservation until analyses. Polymer solution for sludge conditioning was prepared fresh each day. The optimum polymer dose was using capillary suction time (CST) [method 2710G of standard methods (APHA 1995)] and noting the dose giving the lowest response time. A triton type CST apparatus [304-M and 165] was used for this purpose with Whatman 17-HCR as the chromatography paper. The optimum polymer dose for each sludge type was determined after addition of polymer to sludge followed by shearing using a 1/5 HP waring blender for five seconds. Once the optimum polymer dose was determined, sludge samples were dewatered using a centrifuge (Beckman J2-HS Centrifuge or Beckman-Coulter Avanti-JE) at 17,600 x g for 15 minutes. A solids content of 14-18% was obtained. Between six and seven grams of cake solids was placed into 40 mL EPA vials (Fisher brand) for incubation under anaerobic conditions at 25 C for the duration of the experiment. Samples were analyzed on days 0.5, 1, 2, 3, 6 and 11.
Metals were analyzed using an atomic absorption spectrometer after acid digestion by EPA 3050B method. Samples were dried overnight at 105 degree C to completely remove water before acid digestion. Metals were quantified using a calibration curve made for iron and aluminum using respective standards.
Labile proteins were quantified using the Hartree modification of Lowry et.al method after extraction by the method developed in the WERF II project (insert ref). Extraction of samples was performed by first dewatering the sludge samples in case they were in liquid phase. Ten grams of the dewatered sludge cake or biosolids cake was re-suspended in pH 8 phosphate buffer saline solution (PBS). The suspension was sheared for ten minutes at G = 1000/s followed by centrifugation at 3000 x g for 15 minutes. The centrate obtained after filtering through a 1.5 um glass microfiber filter was termed as labile or bound protein. Bovine serum albumin was used as the protein standard.
Chapter 1. Literature review
Volatile sulfur compound odors in the environment
TMA odors in the environment
Properties of select odor compounds of interest
Proteins as precursors for odor forming compounds
Role of shear and polymer
Mechanisms of sulfur odor generation and consumption
Odor detection methods
Role of chemicals in odor production and also as a means for odor abatement
Role of metals
Chapter 2. Inter-relation between trivalent cations and labile proteins and its influence on odor production from wastewater sludges and biosolids.
Results and discussion
Summary and conclusions
Chapter 3. Evaluation of Trimethylamine Generation from Biosolids
Materials and Methods
Results and discussion
Summary and conclusions
Chapter 4. Engineering significance
GET THE COMPLETE PROJECT
Investigating the Role of Various Environment and Process Conditions in Wastewater Sludge Odor Generation