Water. desalination + reuse

| 18 | Desalination & Water Reuse | November-December 2014 projects _________ Al Goodman, Principal, CDM Smith ___ Editor's note: effluent water reuse in the food industry raises inevitable questions about the risk and reliability of wastewater treatment and recovery processes in meeting the required standards. The author looks at how two food processing facilities adopted process water recovery and reuse shows how the benefits can be a maximized and how the challenges live among the detail. TREATMENT OF effluent water from food industry processes for direct and reuse in subsequent food production clearly promises sustainability gains. With those gains come economic benefits and potential marketing opportunities. But they also carry concerns about how consumers might perceive the risk to product quality. In each of the two studies described here, food processing wastewater treatment systems are installed and on-line for the recovery of process water. And the recycled water meets the US Environment Protection Agency (EPA) and Food and Drug Administration (FDA) drinking water quality standards or equivalent. The plAnTs The first facility considered is a corn and potato chips snack foods production plant with a peak flow rate of 2,250 litres/minute. It operates in a desert climate where reuse is an economic and sustainable solution. The designed recovery flow is 75% of the plant's wastewater. The second food facility is a potato processing plant in northwest USA. Design flow is 16 Ml/d with 7 Ml/d of flow treated to be equivalent to drinking water standards for reuse in potato processing. In both facilities, water is recovered and treated to be stored and reused in contact with food products through cooking, washing and sanitation of equipment. And primary clarifier sludge and waste activated sludge from each factory is dewatered by centrifuges and disposed to farmers as animal feed or landfill. There are several processes common to both plants (see box 1) but there are treatment steps that differentiate the facilities. In the snack food producer, oil and grease capture/ segregation is important. Also it is distinguished from the potato processor by its use of carbon dioxide in acidity control; ultraviolet light (UV) disinfection; and granular activated carbon (GAC) to adsorb membrane foulants and remove water discolouration. The potato processor, unlike the snack food plant, uses anaerobic digestion, alkalinity addition, zero-liquid discharge, and enhanced evaporation of reverse osmosis (RO) reject water. Risk consideRATion There are many features for consideration at the outset of planning and designing a reuse system (see box 2). But ultimately they amount to an assessment of risk. Consideration of the risk of failure of each component is crucial as is evaluation of risk due to inaccurate chemical additions; cross-connection of piping systems; operator error; and power failures. There are programs and models for performing analytical risk-assessments for process water treatment and recycling facilities. They quantify risks – including their potential impact – and propose a mitigation approach to reduce or eliminate the risk. This is an iterative process involving plant operations, maintenance, insurance, financial, procurement, and other management staff. It should be performed repeatedly in key phases of the design so mitigation can be fully incorporated in the final system. The models used at each facility were reviewed during the design phases of the project. At each stage plant management decided to accept the risk, or mitigate the risk and modify the design by, for example, adding redundant components. selecTion of TReATmenT TechnoloGies Each facility used similar technologies and components in its treatment systems. But they each incorporated small but significant differences in their approaches to selecting components and processes for water reuse. While both used a request-for-proposals (RFP) approach to pre-qualified and selected consultants for design/build proposals they differed in how they solicited the proposals. Treatment of process effluent water for reuse in food production

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