Ostara s Pond Water Treatment Goal: merging cost effective treatment with high water quality through resource recovery

Transcription

1 Ostara s Pond Water Treatment Goal: merging cost effective treatment with high water quality through resource recovery Authors: Ahren Britton, Don Clark and Ram Prasad (Ostara Nutrient Recovery Technologies Inc.) ABSTRACT Pond water inventories have grown to represent an increasingly significant liability in the eyes of operating companies, as well as environmental regulators around the world recently. As a result there is a new found impetus to find a treatment solution that results in both high quality water and relatively low cost of operation. Over the past 7 years, Ostara has been developing the Pearl processes to recover phosphates and ammonia from wastewater in an economically and environmentally sustainable way through production of the mineral struvite (or magnesium ammonium phosphate) which Ostara markets as Crystal Green fertilizer. More recently Ostara has adapted the process to the treatment of phosphogypsum stack pond water to meet new stringent environmental discharge or reuse criteria, while recovering a high quality, slow release, granular fertilizer product. The process consists of a pre-treatment system for reducing fluoride, silica and calcium levels in order to allow for crystallization/granulation of struvite in the Pearl process (a fluidized bed crystallizer). This is followed by post treatment using a membrane system to achieve targeted water qualities, whether for discharge to the environment or reuse within a phosphate complex. The process has now been demonstrated in the laboratory for 6 different sources from around the world, and at pilot scale for 3 central Florida sites. Ostara is constructing an industrial scale demonstration plant at Mosaic s Riverview facility that is expected to begin operation in late summer Key Words: Pond Water Treatment, resource recovery, water reuse, struvite INTRODUCTION The environmental and economic liabilities associated with pondwater inventories in the state of Florida have been growing over the past years as regulations for the methods of operation, impoundment and treatment as well as environmental discharge criteria become more stringent. As a result of these increasing pressures, many 1

2 operating phosphate companies have renewed their efforts to implement economically and environmentally sound pond water management strategies. During the same timeframe the recovery of phosphates from municipal and food and beverage sector wastewaters has become a growing trend around the world, with struvite production being one of the principal pathways being used. Struvite production has the benefit of recovering both ammonia and phosphate from the wastewater, and converting it into a slow release fertilizer product that is directly useable in horticulture, turf and agriculture. THE PEARL PROCESS Ostara s Pearl process has been in use in municipal wastewater treatment plants since 2007, and is currently operating or being implemented in 8 plants. It is a fluidized bed reactor designed for controlled crystallization and crystal growth of struvite, controllably producing granules of struvite in the 100 to 350 SGN size ranges for targeted markets. Figure 1 shows a schematic representation of the Pearl reactor. Feed wastewater and reagents are introduced into a stream of recycled treated effluent. This blend of fresh wastewater and reagents passes through the fluidised bed of crystals, allowing them to grow slowly over a number of days until they reach the desired size. Ostara markets the produced fertilizer under its Crystal Green brand on a cost/revenue sharing basis with the client sites. Figure 1: Schematic representation of the Pearl reactor POND WATER TREATMENT PROCESS DEVELLOPMENT Pond water or phosphate plant process water presents a number of technical and economic challenges for treatment to emerging environmental discharge standards. 2

3 These include a very low ph, high concentration of dissolved solids, including phosphate, fluoride, sulphate, calcium, magnesium, ammonia, silica, and sodium, along with a wide variety of additional trace metals. This relatively complex mixture has a tendency to form a wide variety of precipitates, gels and crystals which must be managed carefully to allow efficient treatment, and particularly to recover selected components in pure forms. Ostara spent three years of lab and pilot testing to develop a treatment process capable of accomplishing this economically while recovering significant fractions of the phosphate and ammonia in the pond water as struvite crystals. Jar testing Initial testing was carried out in jars/beakers to rapidly test a variety of potential reagent combinations to selectively remove fluoride, silica and calcium from the pond water while minimizing phosphate losses with the associated precipitates. Ultimately Ostara came up with a 3 stage precipitation process which was capable or removing sufficient amounts of the target components while maintaining about 70% of the phosphate in solution. This pre-treated solution was then suitable for struvite precipitation. The next stage was to demonstrate that the struvite crystals could be formed into marketable sized granules. This required pilot testing in one of Ostara s pilot systems that could produce about 10 lbs per day of Crystal Green. Pilot testing Pilot testing and development was undertaken over the course of approximately 18 months at the Jacobs engineering pilot facility in Lakeland, Florida. Initial experiments yielded very high fractions of fine powdered struvite, or fines, with little crystal growth. Further testing showed that there was a relatively narrow set of conditions under which crystal growth would occur with minimal fines formation. This required more accurate control of the pre-treatment conditions in order to reduce the formation of amorphous co-precipitates, and to minimize the effects of silica gels. Once the appropriate pre-treatment conditions were developed, and the Pearl reactor was optimized for crystal growth, the Crystal Green produced was remarkably white, granular and uniform. Figure 2 shows a sample of the product from the pilot operation after optimization, and Table 1 shows the average product composition during pilot operation. Chemical analysis of the product showed that it was nearly pure struvite, and that it would easily meet North American and international fertilizer standards for metals. 3

4 Figure 2: Crystal Green pellets recovered from Ostara s pilot plant. Figure 3: Water quality as it progresses through Ostara s treatment process (raw pond water on left, final effluent on right) The final phase of process development required us to demonstrate that the Pearl effluent could be polished to meet a range of expected environmental discharge or reuse criteria, and to demonstrate that none of the product, by-product or effluent streams exhibited any hazardous characteristics that could adversely affect the process economics or regulatory treatment. The product and by-product solids were tested for total composition analysis (metals and organics), TCLP analysis (metals and organics), and other parameters, and none were found to be Hazardous or exceed any Land Ban standards. None of the organic contaminants were detected in any of the analysis performed and results for total metals and TCLP metals are presented in Table 1. 4

5 Table 1: Crystal Green product analysis from pilot plant Parameter Total Nitrogen (%N) 5.3 Total Phosphate (%P2O5) 30 Total Potassium (%K) 0.1 Magnesium (%Mg) 10 Total Analysis 1 AAPFCO Standards (for 28% P2O5) TCLP Result (mg/l) 2 Arsenic (mg/kg) Barium 1.19 NA Cadmium (mg/kg) Chromium (mg/kg) 0.31 NA Cobalt (mg/kg) NA NA Molybdenum (mg/kg) NA NA Nickel (mg/kg) NA NA Lead (mg/kg) Selenium (mg/kg) Silver 0.06 NA Zinc (mg/kg) NA NA Mercury (mg/kg) TCLP Standard (mg/l) Table 2 shows the water qualities that were achieved using various membrane treatments of the Pearl effluent over a 2 month steady state operating period, while Figure 3 shows visually how the appearance of the pond water is modified through the treatment process. Because of the relatively extensive pre-treatment system upstream of the membranes, relatively little membrane fouling was observed, and that was easily reversible using conventional membrane cleaning solutions. 1 Results in italics were detected at or below the method detection limit used and are reported as the method detection limit (MDL) 2 Results in italics were detected at or below the method detection limit used and are reported as the method detection limit (MDL) 5

6 Table 2: Pearl pilot plant water quality performance (2 month average) Parameter Typical Pond water 3 Pearl Effluent NF Effluent ph Conductivity (us/cm) 22,100 23, TSS (mg/l) 22 NA 0 0 Ammonia (mg/l as N) Fluoride (mg/l) Phosphorus (mg/l) RO Effluent Once we succeeded in demonstrating that we were able to produce both a recovered granular product that meets fertilizer standards, and a treated water quality that could meet any expected environmental discharge or reuse standards, in an economically attractive manner, we were able to secure a contract with Mosaic to host a commercial demonstration of the technology. Demonstration Plant On Feb 28 th 2012, Ostara broke ground on our commercial demonstration plant at Mosaic s Riverview facility. The plant is designed to recover nominally 10 tons of Crystal Green fertilizer per day from a treated flow of GPM of pond water depending on recovery efficiency and pond water concentrations. Figure 4 below shows a 3D model of the demonstration plant being built. The plant is a complete treatment system and fertilizer packaging plant with onsite storage for up to 160 tons of product. Construction is expected to be complete in summer of 2012 with treated effluent and Crystal Green production beginning to be produced in the July-August timeframe. The demonstration plant has been designed to have considerable built in flexibility in order to test and optimize the design, and develop the capital costs of a commercial system capable of treating up to 500 GPM of pond water. 3 Typical Process Water Analysis from Florida Institute of Phosphate Research 6

7 Figure 4: Ostara s demonstration plant at Mosaic s Riverview facility. CONCLUSIONS In response to increasing incentives to treat and discharge accumulated pond water volumes, Ostara has developed a treatment process that economically produces a high quality treated effluent while recovering a significant fraction of the ammonia and phosphate value in the form of a struvite based fertilizer. The process development took place over a 3 year period, and has resulted in a number of patent applications and process improvements. A commercial scale demonstration plant designed for up to 50 GPM of treatment capacity is currently under construction at Mosaic s Riverview facility with an expected start-up in the late summer of Further progress will be reported in the presentation at the conference as construction progresses and results become available. When proven at commercial scale, this process will present the opportunity to treat and discharge or reuse excess pond water volumes on a continuous basis. This would allow operating companies to work towards operating with minimum water volumes required for heat balance, as well as reducing the financial liability associated with water volumes and total dissolved solids (TDS) in the ponds. This could combine to significantly reduce fresh water intake in an active plant, reduce final closure liabilities, and generate a source of revenue to fund leacheate treatment at closure, while recovering about two thirds of the phosphate in the pond water and reducing residual sludge volumes. 7

8 ACKNOWLEDGEMENTS Ostara would like to thank our partners who contributed to making this project a reality; Mosaic, Jacobs Engineering, Noram Engineering, DCR, Moretrench and CMW 8