MEETING WASTEWATER DISCHARGE REQUIREMENTS FOR A SOYBEAN OIL PLANT IN COSTA RICA

Transcription

1 MEETING WASTEWATER DISCHARGE REQUIREMENTS FOR A SOYBEAN OIL PLANT IN COSTA RICA Isabel S. Fung (1) E.I.T., Received her B.S. degree in Civil Engineering from Texas A&M University, College Station, Texas in Ms. Fung has worked in the environmental field since 1995, as a civil and environmental engineer for WSBC Civil Engineers, Inc. in Houston, Texas. Her field of expertise includes municipal wastewater treatment, lift station and force main design, and construction services administration. Raúl León Morales Received his B.S. degree in Industrial Chemistry from the Universidad de Costa Rica in Is the General Manager and Owner of Técnica del Futuro, S.A. and is a Certified Environmental Auditor in San José, Costa Rica. Has taken wastewater courses in operator training, industrial waste treatment and utility management from the California State University - Sacramento. Address (1) : WSBC Civil Engineers, Inc West Loop South, Suite Houston Texas Phone: (713) Fax: (713) address: ifung@wsbc.com ABSTRACT Sequencing Batch Reactor (SBR) technology treats wastewater on a batch basis. The typical operation involves filling a tank with raw wastewater or primary effluent, aerating the wastewater to convert organics into microbial mass, providing a period for settling, discharging (decanting) the treated effluent, and a period defined as idle that represents the time after discharging the tank and before refilling. This project evaluated Sequencing Batch Reactor (SBR) technology to treat wastewater coming from the soybean oil extraction plant to reduce BOD, COD and fats, greases and oils (FOG) for discharge in the city s Publicly Owned Treatment Works (POTW). The refinado comes from the refined/ neutralization process of the plant, with a high COD (close to 7000 ppm) and a FOG of 1600 ppm. The extractoras come from the extraction process with a COD of 1200 ppm and a FOG of 1100 ppm. The goal of the pilot test is to produce an effluent meeting the government standard for a POTW: a COD of 1000 ppm, BOD of 300 ppm and FOG of 100 ppm. A pilot test plant was set up and time parameters were estimated for aeration, settling and decanting cycles. Pilot testing results will be used to determine the final sizing of the wastewater treatment plant. Further tests showed that the extractoras contained less FOG (140 ppm) but a higher COD near 2100 ppm. Results for the refinado showed a higher COD and FOG, due to newly installed equipment in the process. KEYWORDS: SBR, Oil and Greases, Soybean Oil. 20 o Congresso Brasileiro de Engenharia Sanitária e Ambiental 4379

2 ENVIRONMENTAL LAWS AND REGULATIONS IN COSTA RICA Costa Rica has adopted desarrollo sostenible (sustainable development) as its principal element for economic and social development. During the early 1990s, a new constitutional banner was erected declaring every citizen has the right to a clean environment. In 1992, Fauna Silvestre (Wild Fauna) Laws were instituted, regulating untreated wastewater sewage disposal into watersheds and providing an appropriate setting for private industries to improve their wastewater treatment systems. Within the last two years, the new laws have accomplished the following: Establishment of Community Environmental Councils to decentralize control. Requirement that new industries present Environmental Impact Studies prior to starting operations. Establishment of Environmental Courts, where companies are classified and discharge limits are determined. Requirements for all companies and industries to present periodic Industrial Reports to the Government. Seventy percent of Costa Rica s economy is based on agro-industrial production. In this case, food processor and extractor plants for edible oil with discharges larger than 100 cubic meters/day are being required to present the following information to the appropriate authorities: Flows (m 3 /day) ph Dissolved Solids BOD Temperature Settling Solids COD Total Solids Fats, Oils and Greases If these companies do not meet established discharge parameters, they must present a Chronogram of Activities showing how they plan to improve their existing treatment system. MANUFACTURING PROCESS OF THE SOY BEAN OIL PLANT The existing manufacturing process of this particular plant consists of extracting vegetable oil from the soybean. Hexane (a light solvent) is used to extract the oil and is later recovered and reused. Several steps take place, such as refining and acidulation, bleaching, hydrogenation, fractionation, and deodorization to purify the oil and eliminate contaminants that produce unwanted color, odor and taste. The existing soybean oil plant has an acidulation step as indicated in Figure 1. The process consists of treating this product (refinado) in the refining stage with sulfuric acid and water vapor to regenerate the fatty acids, which are later neutralized with caustic soda. 20 o Congresso Brasileiro de Engenharia Sanitária e Ambiental 4380

3 Soybean Grain Extraction Refining Acidulation Products To POTW Figure 1: Existing Process Flow Diagram for Soybean Oil Plant. The process of acidulation is very inconvenient because it is very expensive, and implies the use of dangerous products and water vapor. The existing plant wants to eliminate the acidulation process, and combine the wastes of the refinado and extractoras into a biological treatment system (pre-treatment) that would permit them to discharge its final effluent into a municipal wastewater treatment plant, or Publicly Owned Treatment Works (POTW). The new process diagram would be as follows: Soybean Grain Extraction Refining Products Wastewater Treatment System To POTW Figure 2: Proposed Process Flow Diagram for Soybean Oil Plant. 20 o Congresso Brasileiro de Engenharia Sanitária e Ambiental 4381

4 EXISTING WASTEWATER TREATMENT AT THE SOYBEAN OIL PLANT The existing treatment takes the effluent of the refinado and pumps it into a reactor tank, where the effluent is mixed and vapor and sulfuric acid added. The reactor tank has a volume capacity of 15 cubic meters. The mix is then sent to an oil-water separator where the oils are skimmed and transferred to acid oil stations for further treatment. The remaining wastewater is sent into four (4) neutralization basins with volume capacity of 4 cubic meters (1 cubic meter each), and air is added to improve the mix. Then the effluent goes into four (4) separation basins of 1.95 meters deep where it combines with the flow of the extractoras, and the combined effluent is pumped to the general wastewater collection system. Refer to Figure 3 for treatment plant diagram. Sulfuric Acid Vapor Acid-Oil Stations Pump To POTW Reactor Tank Oil Water Separator Pump Pump Air Neutralization Basins Separation Basins Figure 3: Plan view of the existing Soybean Oil Wastewater Treatment Plant. STUDY OF THE SEQUENCING BATCH REACTOR (SBR) WASTEWATER TREATMENT SYSTEM SBR technology treats wastewater on a batch basis. The typical operation involves filling a reactor tank with raw wastewater or primary effluent, aerating the wastewater to convert organics into microbial mass, providing a period for settling, discharging (decanting) the treated effluent, and a period defined as idle that represents the time after discharging the tank and before refilling. The SBR cycle length is typically varied from three to 24 hours. Rate and time of aeration are scheduled by the process controller or done manually to achieve the specific proper conditions required for the degree of treatment intended. Advantages of the SBR System include: It is well suited for systems with a wide range of flow and/or organic loading. Complete treatment takes place in a single basin. 20 o Congresso Brasileiro de Engenharia Sanitária e Ambiental 4382

5 It can achieve high BOD and suspended solids reductions and can be operated with or without nutrient removal. Provides elimination of a secondary clarifier and return activated sludge pumping. INFLUENT PURPOSE OPERATION FILL ADD SUBSTRATE AEREATION ON OR OFF REACT REACTION TIME AERATION ON SETTLE CLARIFICATION AERATION OFF DECANT EFFLUENT AERATION OFF DISCHARGE EFFLUENT IDLE CYCLE COMPLETE AEREATION ON OR OFF Figure 4: SBR Operation Sequence. 20 o Congresso Brasileiro de Engenharia Sanitária e Ambiental 4383

6 PILOT PLANT TESTING AND METHODOLOGY A pilot test plant was set up using the available equipment in the existing plant. The plant obtained a temporary permit from the City s POTW to discharge raw wastewater into the system, while the reactor tank was put off line for 30 days and converted to the SBR reactor with an adapted air entrance and piping for loading and discharge. The reactor tank also had a mixer. The reactor tank has a base diameter of 2.3 meters with an area of 4.15 m 2, and a usable height of 3.1 meters. An additional tank of 3.5 m 3 was used as an equalization/homogeneous basin. This tank, due to its location, could only discharge 1.8 m 3 per load through gravity. The auxiliary tank is 1.47 meters in diameter and 2.07 meters in height. Valve Equalization Basin Valve Valve Directly to POTW Pilot Plant Effluent Valve Reactor Tank Valve Directly to POTW Air Figure 5: Pilot Plant Flow Diagram. For aeration an air pressure of 5.5 psi was maintained (better turbulence was encountered at this pressure). The aeration coils where located 1 m above the bottom of the tank. Bubble dispersion was created using a mechanical mixer with paddles, which produced good dispersion of bubbles, thus increasing the transference of O 2 to the mass. There are two influents, the refinado, which comes from the refined/neutralization process, high in COD, close to 7000 ppm and Fats, Oils and Greases (FOG) of 1600 ppm. The other one, the extractoras comes from the extraction process, has a COD of 1200 ppm and FOG of 1100 ppm. Table 1: Pilot Plant Influent Characteristics (during Process Stabilization Phase). Parameters Flow (m 3 /hour) 7 7 FOG 1600 mg/l 1100 mg/l COD mg/l mg/l Temperature 41-50ºC 45-56ºC ph o Congresso Brasileiro de Engenharia Sanitária e Ambiental 4384

7 Step 1 - BIOMASS GROWTH AND STABILIZATION PROCEDURE Specialized bacteria were used to accelerate the growth of the needed microorganisms (biomass) which will be responsible for the biodegradation of the contaminant material. Macronutrients (nitrogen and phosphorus) were also used with a specialized mix of micronutrients (patented by a US company). The Reactor Tank was one third filled (1 meter) with viable wastewater sludge. A 24-hour cycle was started with the following cycle times for aeration, settling and decanting: Fill-Aerate- Mixing: React: Settle: Decant (clear liquid): 12 hours (Fill rate should be uniform over the 12-hour period) 6 hours 3 hours two thirds of the reactor tank (2 meters) for a maximum length of time of 3 hours The cycle was repeated for 7 consecutive days (mixing, aeration and decanting) in order to acclimatize the microorganisms to the soybean oil plant loads (refinado and extractoras influents). At this stage it was not necessary to conduct testing or sampling since the microorganisms were being stabilized and acclimated. During decant, it was not necessary to separate effluent from waste sludge. Step 2 - SAMPLE TESTING PROCEDURES Sampling was started for COD, BOD, FOG, SVI, DO and MLSS for the last decant cycle in Step 1, and prior to the first test fill in Step 3. Also COD, BOD and FOG was obtained on raw waste for the first test fill. Step 3 - PILOT PLANT TESTING PROCEDURES The pilot plant testing was started with the goal of maintaining 4,000 mg/l MLSS of viable wastewater sludge and a F/M ratio of 0.2. Two or three cycles (mixing, aeration, settling and decanting) were run at this target value of MLSS and F/M ratio to provide stabilization of the biomass. Sludge wasting was varied to achieve 4,000 mg/l MLSS. The volume of sludge wasted, MLSS, flowmeter readings, times and other parameters outlined in Step 2 were recorded at each cycle. COD, BOD, FOG, SVI, DO and MLSS were sampled and recorded in 2- hour intervals during the fill-aerate-mixing and react cycles (18 hours). The pilot plant was operated during the tests as if it were the actual plant. For each test cycle, clarified liquid waste was decanted and sampled (COD, BOD and FOG) independent of sludge wasting. Waste sludge was tested in each test cycle for same parameters as in Step 2, and volume of sludge wasted and volume remaining in the reactor as in Step 3 were recorded. During the pilot testing, a problem with some newly installed equipment in the soybean oil process took place. After the new equipment was installed, the refinado COD and FOG increased considerably. The extractoras yielded a higher COD (2100 ppm), approximately 2 to 2.5 times higher than ppm showed in Table 1, although FOG was lowered. Refer to Table 2 for the new values recorded. 20 o Congresso Brasileiro de Engenharia Sanitária e Ambiental 4385

8 Table 2: Pilot Plant Influent Characteristics (during Final Testing Phase). Parameters FOG 5200 mg/l 140 mg/l COD mg/l 2100 mg/l PILOT PLANT TEST RESULTS Pilot plant tests were performed for the period between August 10 and September 17, Samples of influent and effluent of the pilot test plant were obtained and chemical analysis of fats, oils and greases (FOG), temperature, ph and COD were evaluated. Temperature: The two influents coming from the plant had temperatures ranging from o C. Cooling was done in the equalization basin so that in any case, the temperature coming into the reactor was between o C. Period for Aug 17 - Sept 3, 1998 Average - : 53ºC Average - : 49ºC Temperature Sample No. Figure 6: Temperature for the and. ph: When the two influents were mixed a ph of 6.5 was obtained. This value was within the parameters where the aerobic reducing microorganisms worked very well. Period Aug 17 - Sept 3, 1998 Average : 6.3 Average : 6.99 ph Sample No. Figure 7: ph for the and. 20 o Congresso Brasileiro de Engenharia Sanitária e Ambiental 4386

9 FOG Removal: Sybron, a biotechnology products company, recommended and provided a specialized seed of microorganisms that metabolizes greases. These microorganisms resisted temperatures of up to 45 o C and eliminated the foam produced by the intermediate greasy acids. Average FOG removal was 94%. Period Aug 10 - Sept 17, 1998 Average removal: 94% ph Influent Decant Sample No. Figure 8: Variations in Influent-Effluent of FOG in the Reactor Tank. COD Removal: Influent and effluent COD was measured as reference parameter, since results can be easily and quickly obtained in the laboratory. Once the biomass was stabilized, the results of the last 7 runs provided 76% removal. In the decant phase, microorganism concentrations of up to 5.4 x 10 6 were found that did not settle, thus making the COD values higher. Percent (%) of removal for Sept 10 - Sept 16, 1998 Average : 76% % Sample No. Figure 9: Percent (%) of COD Removal in the Reactor. PILOT PLANT TEST CONCLUSIONS 1. COD removal can be increased, if : Filamentous strings (microorganism concentration found in the decanting liquid was of 5.4 x 10 6 ) can be controlled. This can be accomplished by creating an anoxic/anaerobic condition during the Fill phase of the SBR. 20 o Congresso Brasileiro de Engenharia Sanitária e Ambiental 4387

10 Improve the design of the reactor in relation to height and diameter. The pilot plant reactor had a depth (3.1 meters) larger than the diameter of the tank (2.3 meters). From case studies evaluated, SBR reactors had diameters between 8 and 10 meters with a depth no larger than 4.5 meters. A wider tank will increase the settling area, since the SBR reactor also serves as a clarifier. Decant: this phase was done using a hose that was inserted from the top of the reactor to a specified depth; this created some turbulence due to the pump suction and part of the sludge was collected in this phase. 2. The soybean extraction plant needs to lower its content of organic loads to reach the initial estimated values. Alternatives are being investigated. 3. The SVI during the period when 76% COD removal was obtained had values of ml/g. 4. It is necessary to lower the temperature of the influents to 40-42ºC. 5. By varying different parameters in the pilot tank, i.e. flow, aeration time, etc., an appropriate design parameters for the final SBR system can be determined. SBR Process To POTW Equalization Basin SBR Process To POTW Figure 10: Proposed Final Wastewater Treatment Plant. Técnica del Futuro, S.A. (TEDEFUSA) is in the process of investigating alternatives to lowering the COD of the two influents in the existing plant, and is evaluating various SBR systems and costs. TEDEFUSA is scheduled to present a final draft report to WSBC Civil Engineers, Inc. for evaluation. No further pilot testing has been scheduled at this time. BIBLIOGRAPHIC REFERENCES 1. US Filter/Jet Tech, Wastewater Treatment Systems Design Manual. 2. MIKKELSON, KENNETH, PhD, AQUASBR Design Manual, Aqua Aerobic Systems, Inc., NORCROSS, KENNETH, MS, Sequencing Batch Reactor An Overview, Jet Tech, Inc., NORCROSS, KENNETH, MS, Performance and Design Considerations for SBR Treatment of Food Processing Wastewater, Jet Tech, Inc., NORCROSS, KENNETH L., MS, SBR Treatment of Food Process Wastewater Five Case Studies, Jet Tech, Inc. 6. Center for Environmental Research Information, Sequencing Batch Reactors Summary Report, Environmental Protection Agency, o Congresso Brasileiro de Engenharia Sanitária e Ambiental 4388

11 7. Institute of Shortening and Edible Oils, Inc., Treatment of Wastewaters from Food Oil Processing Plants in Municipal Facilities, o Congresso Brasileiro de Engenharia Sanitária e Ambiental 4389