Case History: Anaerobic and Aerobic Treatment of Textile Wastes at South Carolina Textile Plants. Introduction

Transcription

1 Case History: Anaerobic and Aerobic Treatment of Textile Wastes at South Carolina Textile Plants Charles C. Ross, P.E. Environmental Treatment Systems, Inc. Atlanta, Georgia John S. Cox, P.E. John S. Cox and Associates, Inc. Pickens, South Carolina William A. Dunn, P.E. Dunn and Associates, Inc. Easley, South Carolina Introduction Bleaching and dyeing operations at textile facilities produce significant amounts of wastewater that can be fairly high in biochemical oxygen demand (BOD,,, chemical oxygen demand (COD), oil and grease (O&G) and total suspended solids (TSS). Wastewater metals concentrations can also be high depending on the type of dyes used. This is especially true for dyes containing copper. Over the years, many textile facilities have had to install wastewater pretreatment systems to reduce their load on municipal wastewater treatment plants or treatment systems for direct discharge to a receiving stream. In many cases, the primary objective is the removal of BOD, andor COD. As illustrated in Table 1, there are a number of sources of wastewater BOD, within a textile facility. Wastewater BOD, and COD concentrations obviously will vary with the types of processes employed, the amount of waster used and the amount of fabric processed. From a pretreatment design point-of-view, this wastewater presents some significant challenges: The high wastewater ph often requires neutralization for most treatment processes. Lint and fiber particles can create clogging problems for pumps and piping. A significant amount of the BOD, or COD is in a soluble form which typically requires biological treatment for removal. A typically low BOD,/COD ratio creates problems for many biological systems. The wastewater typically is nutrient deficient which also creates problems for many biological systems. The wastewater is often quite hot which can create problems with pumping and oxygen transfer in aerobic systems. i

2 Table ' Process Lb. BOD, per 100 Lb. Fabric cesses Desize (woven only) Enzyme/starch StarcWCMC mix PVOH or CMC only Scouring Bleaching Peroxide Hyperchlori te Mercerizing No caustic recovery With caustic recovery Heat setting (synthetic only) I Needless to say, these wastewater characteristics present a challenge to anyone considering the design of a textile wastewater treatment system. This paper provides information on the design and installation of the wastewater pretreatment systems for two different South Carolina textile plants: one an existing aerobic treatment system in operation since 1990 (Case Study I) and the othaa soon to be completed anaerobic treatment system (Case Study 11). Case Study I: Aerobic Pretreatment of a Bleaching and Dyeing Wastewater Background In this case study, the facility is a manufacturer of Schiffli machine-embroidered textiles which results in the generation of wastewater from a number of processes: jet dye, embroidery tenter, fabric tenter, bleach range and singer. In 1990, the facility was required to install a pretreatment system because of the problems the high strength wastewater caused at the local municipal wastewater treatment plant. By 1994, the plant's wastewater had increased in strength resulting in a 30% increase in COD loading on the system and the resulting failure to meet pretreatment requirements. This resulted in an upgrade of the facility which is in operation today. The design 2

3 "-,, parameters of this facility in 1994 are shown in Table 2. Flow, gpd Parameter BOD,, mg/l TSS, mg/l COD, mg/l PH, su Influent Effluent Design Permit' 100, ,000 3, , '3.- System Design In 1 990, the pretreatment system included ph adjustment, screening, flow equalization, aerobic treatment (biotower/roughing filter), clarification, aerobic treatment (activated sludge system with internal clarifier), aerobic sludge digestion and sludge dewatering. In 1994, the facility was upgraded to include a 41'-0" circular clarifier added to the end of the process stream for improving biological solids removal. A description of the existing pretreatment facility is provided in Figure 1 and detailed in Table 3. The ph adjustment system consists of a diffused air mixing system in a 12,000 gallon tank. Sulfuric acid is dosed via a metering pump based on the control signal from a ph controller with an electrode in the tank. The neutralized wastewater is pumped through a lint screen and on into a 60,000 equalization tank. The equalization tank is mixed and aerated using a diffused air system and a recycle line from the pumps transferring wastewater to the biotower. The recycle line has an air eductor in-line for additional aeration prior to the biotower. Phosphoric acid is also added to the equalization tank to provide phosphorous for biomass growth in the aerobic systems downstream. The biotower (roughing filter) consists of a 48'-0" dia. X 22'-0'' high tank shell containing 36,000 ft? of NSW Sessil media for aerobic biomass attachment. Wastewater is distributed through a circular rotary distributor which evenly spreads the wastewater over the media bed below. The media consists of bundled strands of reinforced polyethylene strips which serve to improve wastewater distribution and biomass attachment as the wastewater trickles down from the top of the biotower. Treated wastewater and sloughing biomass from the media collects in the bottom of the biotower and gravity flows through a splitter box and on to a circular clarifier. The splitter box serves to return wastewater back to the equalization tank so that the biotower is in continuous operation even during periods of low plant flow. The clarifier serves to remove biosolids from the wastewater prior to entry into the activated sludge system. J 3

4 - ph Adjustment Lint screening Equalization Process Aerobic treatment- Biotower/roughing filter Description 12,000 gallon tank with diffused air mixing; ph electrode and controller to control sulfuric acid metering pump stainless steel basket strainer 60,000 gallon basin, diffused air mixing, nutrient (phosphoric acid) addition 48'-0'' dia. X 22'-0" high biotower with 36,000 ft3 of NSW Sessil media and rotary distributor I Clarification I 13'-0" dia. circular mechanical clarifier I Activated sludge I Clarification I Aerobic sludge digestion 1 - Sludge dewatering 100,000 gallon extended aeration plant (500 scfin) with three hopper clarifiers 4 1 '-0" dia. circular mechanical clarifier two 50,000 gallon aerobic digesters (250 scfm each) sludge conditioning tank with 30 ft3 plate and frame filter press "\ I The activated sludge system consists of a recycled 100,000 gallon extended aeration plant with 500 scfin of blower capacity. The unit has three internal hopper clarifiers for some biological solids removal for recycle or wasting. The wastewater passes through the activated sludge system and on to a 4 1'-0'' circular clarifier which was added in 1994 to improve biosolids recovery. After clarification, the treated wastewater is discharged to the local municipal sewer. All of the recovered solids from either the activated sludge clarifier hoppers and the last clarifier are either returned to the activated sludge system to maintain the mixed liquor suspended solids concentrations or are wasted to one of two 50,000 gallon aerobic digesters. The clarified sludge from the first clarifier (after the biotower) is also wasted to the digesters. Wasted activated sludge (WAS) in the digesters is stabilized after several days of aeration within the digesters. The digested sludge is wasted every shift and dewatered using a plate-in-frame filter press using a metal salt and cationic polymer to improve separation. Supernatant from the digesters and fitrate from the filter press is returned to the equalization tank. Performance Since 1994, the system has met the discharge requirements of the municipal sewer district as illustrated in the effluent values in Table 4 compared to the permit limits in Table 2. Removals for BOD, were fairly high at 84% followed by COD and TSS removals of 69.7 and 60.0%, respectively. 4

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6 Influent Effluent Parameter Actual' Actual' Flow, gpd 82,330 82,330 BOD5, mg/l 2, TSS, mg1l COD, mg/l 8, 026 2,433 PH, su unknown Removal % N/A NIA The plant reports that the operating cost for the system is in the range of $5.OO per 1,000 gallons treated which includes all electrical, labor, chemical and sludge management costs. The facility generates roughly 40 tons of dewatered solids (1 8% TS) each week for disposal. Operational problems have been minimized over the last three years with the exception of sludge disposal and the ability to maintain high dissolved oxygen levels throughout the system due to the high COD concentrations in the wastewater. Case Study II: Anaerobic Pretreatment of a Bleaching and Dyeing Wastewater Background A new South Carolina textile facility to be built for the bleaching and dyeing of woven fabrics was required by the local regulatory authority to install a pretreatment system so that it would meet certain pretreatment limits (Table 5) soon after it began operation. Because most of the wastewater BOD, and COD was expected to be in a soluble form, the facility evaluated a number of biological treatment technologies including activated sludge and anaerobic treatment to meet these requirements. Since the facility was under construction and not generating wastewater, wastewater from a similar facility was used for anaerobic treatability testing. As indicated in Table 6 and Figure 2, the series of batch anaerobic tests confirmed that anaerobic treatment was a technically viable option with COD removals of 80% and anticipated BOD, removals of >85%. 6

7 Parameter Influent Design Influent Actual' Effluent Permit2 Flow, gpd 125,000 BOD,, msn, 4,500 TSS, mg/l 2,500 COD, mg/l 15, , ,000 1, ,210 NIA :I.- $35 E30 U U =20 n Y cn"10 0 Figure 2. Anaerobic Treatability Test Gas Production t... CUMMULATIVE GAS PRODUCTION 33.5ML... 1 fc ,'.....I ML... t ML... --F-T 2.OML 0 5 IO TIME, DAYS I + REACTOR 1 -c REACTOR 2 i '. -E- REACTOR 6 + REACTOR 1- REACTOR 1 7

8 Fintex none 5.64 na na na 2 na Glucose na na na 33.5 na The anaerobic process was selected for primarily three reasons: 1) High COD loading rates. The wastewater COD concentrations were expected to be as high as 15,000 mg/l which are more conducive to anaerobic treatment when compared to aerobic treatment. The anaerobic process would accommodate higher organic loadings while producing less sludge and requiring lower energy and nutrient inputs. Low sludge production. An anaerobic process will typically generate roughly 10% of the biosolids mass that an aerobic system would under similar conditions. Sludge dewatering and disposal can be a significant portion of the operating cost for a treatment system. 3) Energy costs. Since an anaerobic process does not require aeration, the electrical operating costs would be significantly lower. Furthermore, the wastewater was predicted to be discharged at a temperature of F which is an optimal temperature range for mesophillic anaerobic treatment. Methane gas production from the anaerobic degradation of the wastewater organics would also generate roughly 60,000 cubic feet of methane per day (2.5 million BTUH) which could be used by the plant for an energy source. Design A hybrid anaerobic lagoon (HAL) process was designed and the system contracted for installation based on the following: 1) Loading requirements. The design was based on the results of the treatability tests which indicated most of the treatment would take place within 5 days of contact in 8

9 the lagoon (Figure 2 and Table 6). This indicated that a lagoon with a minimum 10 day HRT would provide ample time for treatment. Based on a design COD concentration of 15,000 mg/l, this would translate into an organic loading rate of 1.5 kg COD per m3 of lagoon volume per day. This loading rate is within the traditional design loadings of an ambient temperature anaerobic lagoon (0.5 to 2.0 kg COD/m3-d) and well within the acceptable range of loading rates for a mesophilic temperature system (>80"F). 2) Solids concentration. The wastewater TSS concentrations were expected to be as high as 4,000 mg/l which could be a problem for high rate anaerobic processes employing fixed film or sludge bed designs due to fouling and low solids retention time. Therefore, a hybrid anaerobic lagoon (HAL) process was selected which would be capable of retaining these solids for further reduction. Furthermore, the large lagoon HRT provides high solids retention times and volume for sludge buildup resulting in infrequent solids disposal. 3) Space. The facility had ample space for the installation of the hybrid anaerobic lagoon. Otherwise, a contact type of anaerobic system with a smaller footprint would have been required. - 4) Flow equalization. The textile plant was required by the local sewer authority to provide a constant, equalized flow 24 hours per day over 7 days per week. The large lagoon basin allowed the accumulation of wastewater during the week for subsequent controlled discharge during the weekend when the plant was not operating. Process Description As illustrated in Figure 3 and Table 7, the pretreatment system is relatively simple. Wastewater is transferred from the textile plant to a ph adjustment tank via a lift station. Sulfuric acid and/or caustic is added based on a ph control setting of roughly 7.0. The tank is mixed with a submersible mechanical mixer. As the wastewater is neutralized, light doses of nutrients in the form of aqua ammonia and phosphoric acid are added to encourage anaerobic cell growth. After neutralization and nutrient addition, the wastewater overflows to the influent end of the anaerobic basin. The wastewater flows through four different cells in the system that are defined by three baffle walls across and down the length of the basin. Anaerobic biomass is kept in suspension within the basin by a sludge recycle system which also serves to return settled sludge from the effluent cell to the influent cells. An effluent recycle system is also provided to return treated effluent back to the ph adjustment tank to mix with the incoming wastewater. As the wastewater is discharged from the basin, the ph is monitored and caustic added if needed to maintain a minimum ph of 6.0. The effluent is then routed through a metering manhole with a Parshall flume for flow monitoring via an ultrasonic flow monitor. The wastewater then 9

10 Anaerobic Lagoon (HAL) Gas recovery Effluent monitoring Sludge handling baffled cells; sludge recycling and effluent recycling Floating covers over each baffled cell to retain gas and contain odors; blower system with controls to transfer gas fiom under covers to flare system ph electrode and controller to control caustic metering pump; Parshall flume and ultrasonic flow meter to monitor discharge to city sludge is allowed to settle and digest within basin for periodic recovery through internal piping for contract dewatering and disposal of biosolids Based on a biomass yield of 0.13 lbs of VSS/lb COD removed, it is anticipated that roughly 1,884 gallons of sludge at 5% TS would be generated each day which would be retained by the sludge zone of the lagoon. Assuming that 25% of the lagoon volume is available for retaining this daily sludge volume, there would be roughly 166 days (>5 months) of sludge storage before removal is required. This predicted sludge wasting cycle is likely low because the very long SRTs (>625 d) will maximize the endogenous destruction of settled biomass, further reducing 10

11 6- IL... n Figure 3: Process Diagram for Case Study!I: Anaerobic Pretreatment System 11

12 the amount of sludge generated for disposal. For the sludge that will be removed for disposal, contact has been made with a local firm for sludge dewatering and disposal. On a contract basis, the firm would periodically come to the site and remove sludge from the sludge draw-off ports inside the lagoon basin, dewater it via a trailer mounted belt press and collect the solids in a bin for subsequent deposition in an approved landfill. Filtrate from the belt press would be returned to the influent headworks of the lagoon. Appropriate documentation including manifests would be maintained during the sludge removal process. ) The system is currently in the final stages of installation and will begin start up the end of April. It is expected that the system will be fully operational by June of References Smith, B., Identification and Reduction of Pollution Prevention Sources in Textile Wet Processing. Pollution Prevention Pays Program, Raleigh, NC. Richardson, S., Conversations and internal reports. 12