Methods for Design and Operation of Reuse Systems through an Integrated Approach to Improve Reliability with Reduction in Cost

Data from various parts of California show that without the optimization of assets for a recycled water system, the cost of recycled water production can exceed $1500 per acre foot (1 acre foot equals 0.3 million US gallons, which is equal to 1250 m3), as compared to $500 per acre foot for raw water that may be purchased and transported from natural sources. These costs are before additional treatment and distribution costs which are incurred downstream as part of the drinking water system. This paper presents alternatives to reduce the cost of producing recycled water from wastewater treatment. It does this by optimizing treatment processes and their configuration to the degree the total dissolved solids have to be managed, along with removal of the traditional and emerging contaminants in the recycled water. In many locations, this can lower the capital and life cycle cost of recycled water from three times the average cost of raw water to two times. With the inclusion and/or optimization of additional storage, it may be possible to operate the recycled water facility continuously and at a higher utilization with less variation in water production in wet and dry years, which can also improve the costs.A key challenge in managing the cost of water supply is the cost of managing the total dissolved solids (TDS) of the recycled wastewater. In the US, the TDS of treated wastewater (without reverse osmosis – RO) exceeds the TDS of natural drinking water sources by 300 to 600 mg/L. Therefore, when the drinking water supply is augmented with treated recycled wastewater, the utility may need a strategy for TDS management in the blended water. Typically, the utility may set a threshold for the blended water in terms of mg/L TDS (500 or 1000 mg/L, depending on geography) or a percent increase in the TDS (such as 10 percent over historical levels). If the blended water TDS exceeds these thresholds, the utility will implement a process to reduce the TDS of the treated wastewater before blending. The most common method for managing TDS is to add an RO system. Because of the high operating and life cycle costs of RO systems, this paper looks at methods to substitute the RO system with a GAC or Ozone-BAF (biologically activated filtration on activated carbon), or hybrid options that may use a BAF on the main stream and RO on a portion of the recycled water stream. By reducing the amount of water treated through RO, the expenditure on energy and anti-scaling chemicals is reduced, as also is the amount of RO reject (concentrate or brine) generated that needs to be disposed. An advanced oxidation and disinfection process may be added downstream of the GAC to reduce any residual emerging contaminants, similar to its application in RO systems.