MEMBRANE PILOT FOR A DIRECT REUSE APPLICATION: ENGINEERING AN MF/UF & RO PILOT. Ignacio Cadena, P.E. Freese and Nichols, Inc.

Size: px
Start display at page:

Download "MEMBRANE PILOT FOR A DIRECT REUSE APPLICATION: ENGINEERING AN MF/UF & RO PILOT. Ignacio Cadena, P.E. Freese and Nichols, Inc."

Transcription

1 MEMBRANE PILOT FOR A DIRECT REUSE APPLICATION: ENGINEERING AN MF/UF & RO PILOT Ignacio Cadena, P.E. Freese and Nichols, Inc. Fort Worth, Texas Abstract Piloting of membrane systems has become standard practice to determine source water specific design parameters (membrane flux, recovery rates, backwash frequencies, chemical cleaning parameters and overall operating parameters). As water scarcity increases in the southwestern portion of the USA, application of membranes for reuse applications is of increasing interest. The Colorado River Municipal Water District (CRMWD), located in West Texas, is confronting these water scarcity issues. The district is moving forward with the first direct blending reuse application of membrane filtration and this case study provides an outlook of the challenges of piloting secondary treated effluent from a municipal wastewater treatment plant (WWTP), along with the challenges of operating two pilots units in series (Hollow Fiber and RO membrane system). The Colorado River Municipal Water District has studied many options for increasing and improving the region s water supply. A study 1 was completed to confirm the feasibility of a two stage membrane filtration system to polish the treated effluent from the City of Big Spring WWTP to a quality suitable for blending into the District s raw water pipeline. In addition to increasing the District s water supply, the design of a Tertiary Treatment plant provides the opportunity to improve the region s water quality. The District has completed the design phase and construction begins the summer of This paper provides an overview of the project, piloting and design challenges, and current status of this project. Introduction The Permian Basin, like much of the Western United States, has been subject to an unprecedented drought for the last 15 years. The District s raw water supply is mainly from three reservoirs on the Colorado River: J.B. Thomas, E.V. Spence, and O.H. Ivie. To date, reservoirs have reached alarming low levels. O.H Ivie is the largest of the three reservoirs at 165,000 acre-feet. This reservoir is currently 29% full. E.V. Spence and J.B. Thomas reservoirs have a volume of 10,500 (2% full) and 9,500 acre-feet (4% full) respectively. The District has been seeking new supplies and alternatives to continue providing a reliable and sustainable water supply to its member and customer cities. 1 Regional Water Reclamation Project Feasibility Study. David Sloan, P.E., Ignacio Cadena, P.E. Freese and Nichols, Inc. (2005)

2 A feasibility study was completed in 2005 to determine the technical feasibility of capturing unused wastewater effluent and providing additional treatment to reclaim it for use as a municipal water supply. Three regional projects were evaluated: one in Big Spring, one in Snyder and one between the Cities of Odessa and Midland. The feasibility study concluded all three projects were technically feasible and the cost of the reclaimed water should be comparable with other sources under consideration by the District at the time. However, the Big Spring project 2 had the fewest obstacles to implementation and appeared to be the most cost effective of the three projects. Project Description The Big Spring Raw Water Production Facility will divert approximately 2.5 MGD of treated effluent currently being discharged to Beals Creek and reclaim it for blending into the District s Spence Pipeline for subsequent distribution and use as a municipal water supply. The agreement between the City of Big Spring and the District is to provide treated effluent from the filtered water channel from the City s WWTP prior to dechlorination. The chlorinated effluent must have a minimum chlorine concentration of 1.0 mg/l. This concentration ensures sufficient contact time to claim a disinfection credit for viruses. The effluent will undergo advanced treatment prior to blending. The first treatment step will be membrane filtration, using microfiltration membrane modules. This step will remove particles remaining from previous treatment of the wastewater and associated turbidity. Membrane filtration will also remove protozoan cysts such as Giardia and Cryptosporidia, as well as some bacteria. Membrane filtration also provides excellent pre-treatment for reverse osmosis, which is the second treatment step. Reverse osmosis will remove most of the salinity in the water and remove a wide variety of contaminants with potential health implications such as viruses and most organic molecules, including pesticides and most pharmaceuticals. By treating the entire flow thorough the RO membranes, potential risks from the water s wastewater origins are greatly reduced. The additional reduction in salinity will improve the overall quality of water delivered to customers. The final treatment step is UV oxidation. Some disinfection is warranted to provide a redundant barrier to any pathogenic organisms which may have breached the membrane treatment processes. UV disinfection is desirable because it does not form any known undesirable byproducts and will leave no residual to react with raw water after blending. Enhancing the UV process with advanced oxidation provides an effective treatment for emerging contaminants which can pass RO membranes. The reject from the low pressure membranes, approximately 150,000 gpd, will be sent back to the Wastewater Treatment Plant. Reject from the high pressure membranes, approximately 230,000 gpd will be discharged into Beals Creek, the current effluent receiving stream. 2 Regional Water Reclamation Project Preliminary Design Report. David Sloan, P.E. Freese and Nichols, Inc. (2007)

3 An estimated cost of $2.38/ 1000 gallons is estimated to deliver treated water into the E.V. Spence pipeline for reuse. This cost includes debt service and operating cost for transmission from Big Spring WWTP to the proposed Reclaim Facility, advanced treatment and disinfection, and transmission into CRMWD s water distribution system. Project Implementation Challenges In Texas, like many other states in the country, the use of membrane treatment is subject to site-specific conditions that include a period of pilot testing to determine design criteria, fouling tendencies, and test strategies for chemical addition, cleaning regimens, and other factors to be considered in the design of the system. A pre-selection of membrane manufacturers was completed for the pilot testing. Five manufacturers were considered in the evaluation: Pall Corporation, Siemens, Koch, GE Water and Layne Christensen were invited to participate and submit a proposal. Some manufacturers did not submit proposals, and two manufacturers were ultimately selected for the pilot study: Pall Corporation and Siemens. Multiple meetings with the state regulatory agency, the Texas Commission on Environmental Quality (TCEQ) were held. We acknowledged that the proposed project represented a new step in water reclamation, and will be subject to additional scrutiny. We determined early that this scrutiny is best managed by proactively supplying information to the TCEQ at key milestones and providing ongoing assurances that public health protection will be foremost in the execution of the project. Given the nature of the project and because the project falls outside of the regulatory framework, TCEQ s endorsement of the project was important to set the design, monitoring, record keeping and operational requirements. A critical component in the project was obtaining a TPDES discharge permit for the reverse osmosis concentrate (RO reject). A TPDES discharge permit was requested by the District and issued by TCEQ August 6, The RO concentrate will be discharged to Beals Creek, a saline creek located 1,500 feet north of the site. A reclaimed water use authorization from the TCEQ was required in accordance with Chapter 210 of the Texas Water Code. For the proposed project, TCEQ indicated the authorization request must come from the City of Big Spring as the reclaimed water provider, with CRMWD as the reclaimed water user. The Chapter 210 authorization was approved by TCEQ on July 3, The proposed blending of purified reclaimed water with other raw water supplies is not directly addressed by the Texas Water Code. However, under the general oversight of drinking water supplies assigned to the TCEQ, the TCEQ Drinking Water Section requested the opportunity to review the pilot testing protocol and the plans and specifications for the treatment facility. No blending requirement was imposed by TCEQ, but the reclaimed water is expected to comprise a maximum of 15% of the blended water in the pipeline.

4 The low pressure membrane filter backwash will be sent to the sanitary sewer. An industrial discharge return permit is required by the City of Big Spring Industrial Pretreatment Program. The backwash stream returned to the wastewater treatment plant represents a significant flow (160,000 gpd), however no adverse consequences are anticipated since the concentrated suspended solids trapped by the membranes will not exceed those of typical wastewater (200 mg/l) as long as the City s TPDES discharge limit for TSS is not exceeded. The industrial discharge permit was approved by the City of Big Spring in April Pilot Study Implementation TCEQ requires a pilot study 3 be conducted to demonstrate the feasibility of the membrane process for every water supply proposed for membrane treatment. The pilot study was conducted by two membrane manufacturers from May to November The pilot study approval from TCEQ was received in May Details of the pilot study implementation process are described below. The membrane pilot study was engineered to provide filtered water for two integrated membrane pilot systems, each equipped with a low pressure system (MF/UF) followed by a reverse osmosis system working together. The piping design includes three major components: forward flow piping, drainage piping and turbidity spike test piping. The forward flow piping includes isolation valves and PVC piping with true union terminations. Each pilot unit included a flow control valve (provided by each membrane system) with feedback from a level sensor located inside the pilot unit feed tank. The level instrument controlled the flow into each pilot unit. A schematic of the piping configuration is illustrated in Figure 1. Figure 1. Membrane Pilot Study Process Diagram 3 Regional Water Reclamation Project Pilot Study Report. Ignacio Cadena, P.E. Freese and Nichols, Inc. (2010)

5 The raw water supply, which was the treated secondary effluent, was a loop system engineered for a continuous flow into the membrane pilot units. Feed water to the pilot units was pumped by a 150 gpm submersible pump installed at the filtered effluent channel. The pump was a heavy duty pump suited for continuous operation and capable to overcome approximately 25 feet of total dynamic head (TDH). Excess water from the forward flow pump was returned to the effluent channel. Each hollow fiber membrane pilot had the flexibility to control the amount of water into the feed water (raw water) tank through an open-close valve. Both systems were provided with chlorinated effluent at a rate of 60 to 100 gpm. A 4-inch vent stack was designed to simplify the feed water system that overflows into the manhole in the event of valve malfunction or if both pilot units were down for maintenance, cleaning or during the clean in place (CIP) procedure. A detail of the raw water feed system including the vent stack and the raw water pump is shown in Figure 2. Figure 2. a) Raw water feed vent stack, b) Raw water pump & return line at effluent channel The drainage system included PVC piping to collect all the residual flows generated from the treatment process. The residual flows from the pilot units were: strainer backwash, filtrate stream, membrane backwash, maintenance wash, permeate stream, spent chemicals, neutralized flow and the miscellaneous drains. The drainage pipe from the pilot units flowed by gravity to an existing manhole. Drainage from this manhole flowed by gravity to the head of the WWTP. The manhole cover was replaced with a plywood cover with two 4 inch diameter perforations to allow the drain pipes to discharge into the manhole. A membrane pilot study protocol was submitted to TCEQ for review in July 2008 and the approval letter was received in February The purpose of the protocol was to ensure that the pilot study contained the elements needed by the TCEQ to approve the construction of the proposed membrane filtration facility. The objective of the protocol is to evaluate the efficiency of the proposed treatment process for this application and generate field operational data,

6 including recovery rates, evaluation of flux, backwash frequency and pressure requirements for both membrane systems (MF/UF and RO) together. The pilot testing protocol submitted to TCEQ required a turbidity spike test during the third phase of the pilot study. The proposed turbidity spike test was a 4-hour event where the turbidity through the pilot units was required to be increased to 50 NTU or more. The purpose of this test was to evaluate the performance of the membranes under simulated high turbidity conditions. This test was never completed as problems with the WWTP WAS pumps inhibited the wasting of solids, allowing some solids to carry over the clarifiers into the filters. Turbidities from 60 NTU to > 100 NTU were recorded for an 18-hour span. The operation data from the pilot units during this period was submitted and approved as the turbidity spike event. Pall operated an automated test unit equipped with a single UNA-620A hollow-fiber MF membrane. Siemens operated an XP2 pilot machine, utilizing a single L20V oxidation-resistant membrane module. Both the membrane pilot units and chemicals used in the pilots conform to American National Standards Institute/National Sanitation Foundation (ANSI/NSF) Standard 60 and 61. A summary of the MF/UF Pilot Study test results is described in Table 1. Table 1. Low Pressure membranes Test Results Membrane Manufacturer Pall Corp. Siemens Flux rate, Phase I 20 C) Flux rate, Phase II 20 C) Flux rate, Phase III 20 C) Avg. TMP, Phase II 20 C) Average filtrate turbidity (mntu) The Pall RO pilot unit consisted of a manual Pall RO pilot configured in a 2/2/1/1 array. The RO unit was fed from an 1100 gallon break tank that was filled by the MF system. A low pressure booster pump provided flow through a 5 micron pre-filter prior to entering the high pressure pump. A level indicator was used to pause the RO unit if tank level was depleted. Sodium bisulfite and antiscalant were dosed upstream of the pre-filter. Table 2. RO membranes Test Results Membrane System Pall Corp. Siemens Recovery Percentage 75 % 67 %* Avg. feed conductivity (ms/cm) Avg. permeate conductivity (µs/cm) *Limited by the 1/1/1 array used during the pilot study.

7 The Siemens RO pilot unit consisted of a semi-automatic unit configured in a 1/1/1 array. The RO unit was fed from a 1000 gallon break tank that was filled by the UF system. Siemens replaced the use of sodium bisulfite with ammonium chloride. Antiscalant was also dosed upstream of the pre-filter. A low pressure booster pump was used to flow through a 5 micron cartridge prefilter prior to the high pressure pump. A summary of the RO Pilot Study test results is described in Table 2. Pilot Study Challenges The District and the manufacturers experienced several challenges during the pilot unit startup. The first issue occurred when sand from the tertiary filters was found clogging the strainers. It was believed that some sand particles may have passed through the 300 micron strainer and breached into the membrane housing. Sand from the tertiary filters had accumulated in the bottom of the effluent channel and the sand was pumped into the pilot units. The feed water pump was installed approximately 6 inches from the bottom of the channel, allowing the sand to be drawn into the suction. The hollow fiber membranes were replaced due to the risk of sand entering the membrane housing as the effect of sand during backwash would act like sand paper in the surface of the hollow fibers, damaging membranes and jeopardizing the test. Another challenge encountered during startup was that the chlorine dosage of the treated secondary effluent was not flow-paced but was fed at a constant rate. The fluctuation of free chlorine residual at any given time would go from 1 mg/l to 10 mg/l. This chlorine dosing fluctuation created problems estimating the chlorine concentration in the filtrate water entering the break tank, hence the RO feed. The RO feed water needed to be dechlorinated prior to the RO element to avoid damaging the RO membrane elements. A constant feed of 10 mg/l of sodium bisulfite was fed to avoid any chlorine residual beyond the intermediate (break) tank. The turbidity spike event was planned to challenge the membrane systems with unforeseen conditions. The goal was to create a scenario where turbidity was spiked to 50 NTU for a period of 4 hours. A WWTP plant upset coupled with the sand filters going out of service raised the turbidity in the raw water over 80 NTU for approximately 18 hours. This issue worked in the District s benefit as it allowed the membrane system to be operated under a worst case scenario. Project Status The selected membrane system design is complete. The selected membrane manufacturer was Pall Corporation. The project bid phase was completed in March 2011 and is expected to begin construction early in the summer of It is expected the construction of the 1.78 MGD raw water production facility will be completed in the fall of Conclusions The Colorado River Municipal Water District will continue to confront water scarcity issues by exploring innovative technology and ways to treat available water resources to provide a viable solution to the water supply issue in the region. As water scarcity increases in the

8 southwestern portion of the USA, application of membranes for reuse applications is increasing in interest. The District is moving forward with the first direct blend reuse application of membrane filtration/reverse osmosis in the City of Big Spring, Texas. The challenges to developing treated effluent as an alternative water supply seemed difficult to overcome, but being proactive working with the regulatory agency proved to be a very gratifying experience. TCEQ was excited about the project and their willingness to work with the Engineer and the District is making this project a success. Based on the experience of running both pilot units with secondary effluent from the Big Spring WWTP for the duration of the pilot study, including the effect of a turbidity spike, we concluded the two-stage membrane system is an attractive and reliable treatment scheme for CRMWD.