MEMBRANE BIOREACTORS (MBR) AND REVERSE OSMOSIS (RO) FOR LEACHATE TREATMENT. Abstract

Transcription

1 MEMBRANE BIOREACTORS (MBR) AND REVERSE OSMOSIS (RO) FOR LEACHATE TREATMENT Kevin D. Torrens, BCEEM, Brown and Caldwell, 2 Park Way, Suite 2A, Upper Saddle River, NJ KTorrens@Brwncald.com, Ph: Abstract The solid waste industry has been experiencing a shift in the importance of leachate management in recent years. Regulatory agencies, such as USEPA, are continually evaluating emerging containments for regulation such as Endocrine Disrupting Compounds (EDCs) and Pharmaceutical and Personal Care Products (PPCPs) that are present in leachate. Advanced treatment is often needed to comply with discharge requirements. The MBR process provides a high quality effluent suitable for direct treatment by other advanced technologies such as nano-filtration (NF) or reverse osmosis (RO). RO has been applied for direct treatment of raw leachate, tertiary treatment after ultrafiltration (UF) as well as for tertiary treatment after biological processes including SBRs and MBRs. Systems have been installed globally. Four pilot studies were conducted to evaluate the MBR process for leachate treatment. Both hollow fiber and cross-flow systems from several vendors were evaluated. One system also includes application of RO following the MBR process as a polishing step. Data from these systems highlight biological performance, capabilities, effluent quality and operational considerations such as membrane flux, fouling and trans-membrane pressure (TMP). Results of the pilot studies for key parameters of interest for leachate indicated typical flux rates of 7-13 gallons per square foot per day (gfd) for hollow fiber systems were achievable with maintenance cleaning frequencies of 4-14 days required.. The data demonstrate excellent performance with effluent quality better than that required for direct discharge to surface water under the Federal categorical requirements for landfills (40CFR435). Introduction The solid waste industry has been experiencing a shift in the importance of leachate management in recent years. Leachate management now accounts for approximately 33% of landfill operating budgets according to some sources 1. Additionally, more stringent discharge requirements for nitrogen, chlorides, total dissolved solids (TDS) and chemical oxygen demand (COD) are being implemented. Regulatory agencies, such as United States Environmental Protection Agency (USEPA), are continually evaluating emerging containments for regulation such as Endocrine Disrupting Compounds (EDCs) and Pharmaceutical and Personal Care Products (PPCPs) that are 1 Walker, Tony, Republic Services EREF Leachate Summit, Philadelphia PA, October 9,

2 present in leachate 2. In addition to these regulatory pressures, many landfills are experiencing increased leachate volumes and/or contaminant concentrations as a result of past operating practices as well as greater pressure on landfill gas collection (for both regulatory compliance and economic reasons). The resulting increased loadings, coupled with greater leachate variability and more stringent regulations, has resulted with the need for more robust and advanced treatment processes that can respond to the current and future requirements in an economical fashion. Biological treatment of municipal solid waste (MSW) landfill leachate has been used successfully for both pretreatment (e.g. discharge to Publicly Owned Treatment Works (POTW)) and direct discharge (to receiving waters) for removal of organic and nitrogenous contaminants. Sequencing Batch Reactors (SBRs) are perhaps the most common biological leachate treatment process given their relative simplicity and ability to respond to varying leachate conditions. Membrane processes such as the membrane bioreactor (MBR) are now being successfully applied for leachate treatment and are generally able to provide better effluent quality as an SBR. The MBR process combines biological treatment with membrane filtration (ultrafiltration or UF) for solids separation. The process avoids the need to rely on sludge settling by gravity, as used by SBRs to separate biomass solids from treated leachate. Additionally, the MBR process can operate at sludge concentrations of 2-3 times that of conventional systems, thus, reducing system footprint and also providing a high quality effluent suitable for direct treatment by other advanced technologies such as nano-filtration (NF) or reverse osmosis (RO). RO has been applied for direct treatment of raw leachate, tertiary treatment after UF as well as for tertiary treatment after biological processes including SBRs and MBRs. Application of RO following an SBR requires intermediary treatment components including a decant tank and prefiltration (e.g. media, membrane, cartridge) to manage membrane fouling. Application of RO following an MBR does not require any intermediate processes given the MBR effluent quality. RO has been applied for direct treatment (with pre-filtration) of raw leachate in some cases with varying success. MBR Process Comparison The MBR process couples ultrafiltration membranes as a key process component along with biological treatment as is found in conventional leachate treatment processes such as SBRs. Use of the UF membrane (configured as hollow tubes or flat sheets with either inside-out or outsidein permeate flow) provides a physical barrier to the biological organisms that perform treatment. Physical separation of the biomass from the aqueous phase eliminates sludge settling problems often associated with conventional biological processes and also provides a more diverse biological population due to complete organism retention (due to decoupling of sludge settlability for solids retention). Importantly, the UF membrane allows the MBR to operate at higher sludge concentrations which allows for comparable treatment with a footprint of 2 to 3 times less. Lastly, the UF quality is very high and is suitable for application to tertiary treatment processes such as activated carbon or RO without intermediary filtration processes that are required with conventional systems that rely on gravity separation. For example, poor sludge 2 Preliminary characterization of the Pharmaceutical content of MSW leachate from Three Landfills, Behr, Richard; Stahler, Deborah; Pistell, Anne; Dept. of Environmental Protection, Maine. 2

3 quality due to filamentous growth or upsets can result in poor sludge settling characteristics. The poor sludge settleability results in carry-over of suspended solids to the effluent. This is fully mitigated by an MBR since it provides a physical barrier (typically about a 0.4 um pore size) to prevent release of suspended solids. An overall comparison of SBRs to MBRs is show in Table 1. Table 1 MBR Process Comparison Conventional Biological Treatment (SBR, Activated Feature MBR Sludge, IFAS, Fixed Film) Footprint + - Capital Cost - + O&M Cost - + Effluent Quality + - Operational Complexity + - Process Stability + - Flexibility + - Treatment of emerging + - containments Easy of addition of tertiary + - treatment Improved treatment of + - difficult to degrade compounds Impact of poor sludge quality + - MBR Pilot Study Results Four pilot studies were conducted to evaluate the MBR process for leachate treatment. Three were conducted on raw leachate while one was conducted on a combined flow of domestic wastewater and leachate as would be found in a POTW. Additionally, the pilot studies represent membrane systems provided by three different vendors. Three systems include both nitrification and denitrification. One system also includes application of RO following the MBR process as a polishing step. Results of the pilot studies for key parameters of interest for leachate are show in Table 2. Data from these systems highlight biological performance, capabilities, effluent quality and operational considerations such as membrane flux, fouling and trans-membrane pressure (TMP). The data demonstrate excellent performance with effluent quality better than that required for direct discharge to surface water under the Federal categorical requirements for landfills (40CFR435). Additionally, the absence of suspended solids allows for application of tertiary treatment processes, such as activated carbon or reverse osmosis, should they be required to comply with more stringent discharge limits such as those based on water quality criteria. 3

4 Table 2 MBR Pilot Comparison Data Parameter Site A Site B Site C Site D Wastewater Leachate Domestic + Leachate Leachate Leachate Membrane Type Hollow Fiber, Hollow Fiber, Hollow Fiber, Cross-Flow Submerged (Vendor A) Submerged (Vendor A) Submerged (Vendor B) External (Vendor C) MLSS (mg/l) 10,000 8,551 7,133 15,000 Membrane Flux (gfd) Membrane 4 days 4-7 days 14 days 7-14 days Maintenance Cleaning Frequency Projected Membrane 3 mos. 3 mos. 3-6 mos. 3-6 mos. Recovery Cleaning Frequency Eff. % Rem. Eff. % Rem. Eff. % Rem. Eff. % Rem. COD mg/l 1, % % 2, % 1, % BOD5 mg/l 9.1 NA % 5.0 NA % TKN mg/l % % % % NH3-N mg/l % % % % TDS mg/l 11, , , , % TSS mg/l 0 100% 0 100% 100% 100% 0 100% Fecal Coliform MPN /100 ml 0 100% 0 100% 100% 100% 0 100% Color PCU 4, % % 1, % - - TMP provides an indication of membrane fouling and recovery after cleaning. Results from Site B are presented in Figure 1 and indicate TMP was reasonably steady over a range of membrane flux rates and well below maximum allowable TMP levels. These data suggest limited fouling at the flux rates evaluated. Note that the flux rates typically used for leachate are well below those typical for domestic wastewater applications due to the significantly more complex matrix of leachate in terms of organic and inorganic constituents that can contribute to fouling. 4

5 9/22/2008 9/29/ /6/ /13/ /20/ /27/ /3/ /10/ /17/ /24/ /1/2008 NH3-N, mg/l Temperature, Deg C POST Anoxic ph Impact TMP (psi) 8 gfd 9 gfd 11 gfd 13 gfd Figure 1. Site B Membrane Performance Flux Rate (gfd) and TMP (Transmembrane Pressure) 0 17-May 6-Jun 26-Jun 16-Jul 5-Aug 25-Aug Figure 2 presents results of ammonia removal as a function of temperature. Ammonia removal is reduced at lower temperatures due to reduced nitrification rates. Reduced temperatures will also result in lower flux rates through membranes. Accordingly, winter temperatures are used for biological and membrane design and sizing. Figure 2. Site B Impact of Temperature on Ammonia-N Removal Fouling TMP Limit: 9 psi Ammonia-N (Permeate) Temperature (Raw Leachate) Temperature (Bioreactor) Time Period 5

6 Tables 3 and 4 compare key treatment process characteristics for a typical leachate for an MBR system and SBR system. Table 3 is for a 300,000 gallon per day (gpd) system with biological treatment only while Table 4 is a 32,000 gpd system with RO for tertiary treatment. Costs (AACE Class 5) are comparable between the processes when the additional treatment systems required for tertiary treatment are considered for the SBR option. For biological treatment only, the SBR option is still more costly to achieve a similar effluent quality to an MBR due to effluent suspended solids polishing and disinfection requirements. Table 3 MBR and SBR Cost Comparison Equipment Cost 1 MBR SBR Tankage $900,000 $1,800,000 Membrane System $1,1000,000 NA Biological Air Blowers $252,000 $252,000 Diffusers $260,000 $260,000 Mechanically Drive Weir NA $202,500 Effluent DAF 2 NA $222,500 Sand Filter NA $254,000 Chlorination (Disinfection) NA $77,000 Other Costs Electrical and Instrumentation $165,000 $144,000 Contractor Overhead and $502,400 $613,600 Profit Engineering and Construction $251,200 $306,800 Management Start up and Commissioning $25,125 $30,680 Contingency $628,000 $767,000 Total Capital Costs $4,083,725 $4,903,080 Operating Costs (5 year) Power $1,796,251 $1,845,104 Total 5 Year Costs 3 $5,879,976 $6,775,184 1 Equipment costs includes installation 2 DAF- Dissolved Air Floatation 3 Costs include total capital and 5 year operating costs 6

7 Table 4 Treatment Process Comparison with RO Criteria Units SBR/RO MBR/RO Design Average Daily Flow gpd 20,000 20,000 Design Peak Daily gpd 32,000 32,000 Flow BOD mg/l 6,270 6,270 TSS mg/l Ammonia mg/l 1,190 1,190 Boron mg/l Iron mg/l MEK mg/l 7 7 Description SBR/DAF/Filtration, MBR/RO RO Tankage Volume 2-3X 1X Sludge Concentration mg/l 4,000 12,000+ Building Size Larger (25-50%) Smaller Site Area More (25-50%) Less Complexity More Less Staff Same Same Reliability Lower Higher Estimated Lower Higher Implementation Time Equipment Cost $ $3,295,000 $2,943,000 O&M Cost $/gal $0.036 $0.033 The MBR offers advantages in terms of smaller footprint and less complexity (considering tertiary treatment requirements). While the MBR process offer potential advantages as compared to conventional biological treatment systems, they are not a panacea and are not the best choice for all applications. They have been proven in leachate applications both in the US and worldwide and are worthy of consideration given their attributes and ability to respond to changing conditions and limits. Reverse Osmosis RO has been applied for treatment of leachate for more than 15 years. Applications have included direct treatment of raw leachate as well as polishing steps following pretreatment. Pretreatment has included physico-chemical processes as well as pre-filtration (e.g. UF) and biological processes including SBRs and MBRs. Direct treatment of leachate by RO has been applied successfully in Europe and Asia, and to a significantly lesser extent in the US, using disc tube (DT) and spacer tube (ST) type membranes. Spacer tube type membranes are able to operate at high pressures (up to 1,100 psi leachate stage and 1,800 psi concentrate stage) thus allowing operation at high TDS and fouling conditions. Figure 3 presents a typical flow scheme and installation for a single stage system. Table 5 presents RO performance data for two direct leachate treatment systems in Germany. RO 7

8 systems (ST and spiral wound configurations) have been successfully applied following pretreatment. Pretreatment offers numerous advantages by reducing potential foulants (organic and inorganic) to the membranes. The tradeoff is more complex treatment processes. The benefits of reduced fouling (such as longer membrane life and reduced cleaning) should be weighed against the added reliability, cost and complexity of systems employing pretreatment. Pilot testing is highly recommended, particularly for direct treatment applications. Table 6 presents data from a pilot study using RO following an MBR and shows the raw leachate quality as compared to first stage RO permeate.. Figure 3. Typical Direct RO Treatment Process 8

9 Table 5 Representative Spacer Tube RO Performance Treating Raw Leachate Landfill Schönberg, Parameter Unit Germany Rejection [%] Houthalen, Germany Rejection [%] Conductivity µs/cm 99.63% 99.57% COD mg/l 99.73% 99.28% BOD 5 mg/l 99.96% 99.83% Ammonium NH + 4 -N mg/l 99.54% 99.24% Total P mg/l 99.37% 99.19% Sodium Na mg/l 99.84% 99.84% Potassium K mg/l 99.83% 99.80% Chloride Cl mg/l 99.89% 99.85% 2- Sulfate SO 4 mg/l 99.74% 99.47% Heavy Metals mg/l 99.19% 99.39% Hydrocarbons mg/l 99.67% 99.55% 9

10 Table 6 Laboratory analytical test results for Leachate Stage RO Removal Rates Following MBR. PARAMETER UNIT Raw Leachate 1-Stage RO Permeate 1-Stage RO Efficiency TDS mg/l % BOD5 mg/l % COD mg/l % Alkalinity mg/l % Surfactants MBAS mg/l % Aluminum mg/l 1.64 < 0.1 >99% Arsenic mg/l 0.11 < 0.02 >99% Barium mg/l 0.25 < 0.01 >99% Beryllium mg/l < >99% Boron mg/l % Cadmium mg/l < >99% Calcium mg/l % Chloride mg/l % Chromium mg/l 0.74 < 0.01 >99% Copper mg/l % Fluoride mg/l 2.22 < 0.1 >99% Iron, Total mg/l % Magnesium mg/l % Manganese mg/l % Nickel mg/l % Total Kjeldahl-Nitrogen mg/l % Ammonia-Nitrogen mg/l as N % Nitrate mg/l as N % Total Phosphorus mg/l as P % Phosphate, Ortho mg/l as P % Potassium mg/l % Sodium mg/l % Strontium mg/l 0.56 < 0.01 >99% Sulfate mg/l %* Sulfide mg/l 74 < 0.1 >99% Zinc mg/l % *Sulfate is added to the system through front end ph adjustment with sulfuric acid. Of particular importance with treating leachate through RO is scaling potential due to species of concern such as calcium, silica, strontium, sulfate, barium, iron and phosphorus. Antiscalant software can be useful for evaluating alternative operating conditions (e.g. ph, temperature etc.,) that can influence scaling potential while also providing estimated antiscalant doses for comparison of chemicals and RO operating conditions. 10

11 Summary MBR and RO technologies are viable options for leachate treatment where a high level of treatment is required to meet discharge requirements. Key advantages and challenges of membrane technologies for leachate treatment include: Reduced footprint compared to conventional technologies; Eliminates reliance on gravity separation of solids (biological or chemical derived); High effluent quality to meet current and future discharge limits; Piloting is needed for RO due to site specific fouling potential. MBR piloting is often not required; Modeling of projected RO performance and scaling potential is highly recommended; Pretreatment before RO is needed to reduce fouling potential; Qualified personnel are required for efficient operation, and MBR and RO technology is applicable for leachate treatment and offers a number of advantages as compared to conventional technologies; however applicability is site specific. 11