Salinity Impacts to Water Recycling in Southern California

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ABSTRACT
In 2000, U.S. Bureau of Reclamation (Reclamation) and twelve cost sharing partners began the Southern California Water Recycling Projects Initiative (Initiative), which is a multiyear planning study. This study was established to assist local agencies in their recycled water planning for specific projects as well as addressing regional issues that may influence water recycling in southern California. One of the main objectives of the regional component is to assess and analyze potential future brine/concentrate management issues at a regional level. This issue is of particular concern to agencies in southern California that produce recycled water because of the potential salinity increases in water supplies and increasing discharge regulations.

High salinity in reclaimed wastewater affects all users of reclaimed water including agriculture, horticulture, groundwater replenishment, and industry. Salinity in reclaimed water can interfere with reuse if it results in the water not being able to meet customer water quality needs and expectations or by not meeting regulatory requirements. The degree of restriction to reuse varies from none to severe, depending on the constituent concentration and the reuse application. The major salinity-related problems faced by reclamation agencies in southern California include:

  • Salinity increasing in imported water supplies
  • Ability to meet groundwater basin plan objectives and stream discharge requirements
  • Need for brine management/disposal facilities
  • Need to protect and improve local groundwater basins
  • Provide reclaimed water users with high water quality

This paper will discuss the analysis, results, and information accumulated to date as part of the Initiative project including the effect of potential increases of salinity on water reuse at a regional level such as:

  • How much projected reuse can be expected in the region?
  • Who are the primary users of reuse water?
  • What are the cost impacts due to salinity increases to different reuse types?

INTRODUCTION
Numerous factors contribute to salinity in recycled water in southern California including imported potable water sources and salts entering with each cycle of urban use for residential, commercial, or industrial purposes. In southern California, the capability to continue to use recycled water might be predicated upon the management of the long-term salt imbalance. Management of the salt imbalance is key because as salinity increases, irrigation water use increases to flush out salts that accumulate in the root zone, and industrial users incur extra costs for cooling towers, boilers, and manufacturing processes. In addition, groundwater recharge and saltwater intrusion barrier recharge also can be affected when source water quality does not satisfy regulatory requirements (i.e., Basin Plan Objectives). A map of the southern California Study Area is shown in Figure 1.

STUDY METHODOLOGY
A salinity model was developed to asses the average “annual” impacts based on salinity increases in recycled water. The model uses mathematical functions that define the relationship between TDS and each affected type of recycled water users, such as the specific crop yields, cost for additional irrigation water supplies, industrial and commercial water uses, and cost for desalination in order to satisfy regulatory requirements for groundwater recharge. The result of the analyses is a set of cost functions that depict the additional cost for various types of recycled water users to offset impacts of increasing salinity.

The analytical framework used to evaluate the impacts of salinity on users of recycled water consists of the following elements:

  • Identification of sources of recycled water, available flows and salinity concentrations
  • Grouping of users of recycled water into categories based on water quality (TDS) requirements
  • Establishment of cost models due to increases in salinity including costs resulting from salinity removal based on reverse osmosis (RO) treatment and costs resulting from increased use of water to manage salinity levels
  • Review of cost results and summarize

Cost models are developed for excess irrigation and cooling water use that is required with increasing salinity in reuse water. Cost models also were developed for desalination for those instances when lowering the TDS levels is required to satisfy customer or regulatory water quality requirements. RO was selected as the basis for the cost impact analysis because it is used at several groundwater desalting projects and water recycling facilities (with advanced treatment)currently operating in southern California. Cost impacts were evaluated on a regional basis.

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