Water Conservation, Reuse, and Recycling: Proceedings of an Iranian-American Workshop (2005)

James Crook and Robert S. Jaques

The Monterey Regional Water Pollution Control Agency (MRWPCA) began facilities planning to provide wastewater management services to northern Monterey County, California, in 1975. At that time, water reuse was considered to be an important element in the planning process. The Salinas Valley is an agricultural region in northern Monterey County where a wide variety of market crops are grown. Heavy agricultural and municipal groundwater demands beginning in the 1940s led to the development of severe groundwater overdrafting of the underlying aquifers, resulting in seawater intrusion from adjacent Monterey Bay. The intrusion front was advancing inland at a rate of approximately 150 meters/year (m/yr) (500 feet/year [ft/yr]). High salt levels in groundwater caused wells near the coast to be abandoned, and agricultural water supply wells and some community drinking water wells were threatened. This was a major factor in the decision to develop a regional wastewater management plan to provide reclaimed water for food crop irrigation in the Salinas Valley. By using reclaimed water for irrigation, growers could discontinue pumping from their wells, thus alleviating overdrafting of the groundwater. The wastewater management plan included eliminating nine older wastewater treatment plants by constructing a single centralized regional treatment facility. Figure 1 depicts the water reuse scheme.

A seven-year agricultural reuse demonstration study conducted at Castroville, California, initiated in 1976 and completed in 1987, determined that filtered, secondary effluent meeting a total coliform limit of 2.2/100 milliliters (ml) was acceptable for the spray irrigation of food crops eaten raw. During the study no

pathogenic organisms were detected in the reclaimed water, and spray irrigation with reclaimed water did not adversely affect soil permeability, did not result in heavy metal accumulation in the soil or plant tissue, and did not adversely affect crop yield, quality, or shelf life.

The 114,000 cubic meters per day (m³/d) (30 million gallons per day [mgd]) regional wastewater reclamation facility was constructed adjacent to a regional secondary treatment plant to provide tertiary treated reclaimed water for agricultural applications. A distribution system to serve 4,800 hectares (ha) (12,000 acres [ac]) was constructed to deliver the reclaimed water. The regional wastewater reclamation facility began delivering 76,000 m³/d (20 mgd) of reclaimed water for food crop irrigation in 1998. Reclaimed water is used to irrigate lettuce, celery, broccoli, cauliflower, artichokes, and strawberries. The water reclamation facility and distribution system are collectively known as the Monterey County Water Recycling Projects (MCWRP).

The Monterey Wastewater Reclamation Study for Agriculture (MWRSA), initiated in 1976 and completed in 1987, was an important step in the planning process for the MCWRP. A Task Force composed of representatives from federal, state, and local governments, the academic community, farm advisors, and local growers provided guidance in the planning and conduct of the study. The California State Water Resources Control Board and the U.S. Environmental Protection Agency (EPA) provided funding for the study, which cost a total of $7 million. The goal of MWRSA was to assess the safety and feasibility of agricultural irrigation using reclaimed water to irrigate vegetable crops that may be eaten raw. It included a five-year demonstration project.

The tertiary treatment plant used secondary effluent from the existing Castroville Wastewater Treatment Plant as influent to the pilot tertiary treatment plant, which had two parallel treatment process trains as follows:

1. Up to 57 m³/d (0.015 mgd) was treated using the treatment train specified in California’s Wastewater Reclamation Criteria for reclaimed water used to irrigate food crops that could be consumed raw (secondary treatment, chemical coagulation, clarification, filtration, and disinfection); and

2. Up to 890 m³/d (0.24 mgd) was treated using the direct filtration process (secondary treatment, low dose coagulant/polymer addition, filtration, and disinfection), which provided less extensive treatment than that specified in the California water reuse criteria.

In order to conform to California’s water reuse criteria, both treatment trains were required to have a chlorine residual of at least 5 milligrams per liter (mg/l) after a minimum chlorine contact time of 90 minutes. In addition, the turbidity of the wastewater prior to disinfection was required to be 2 nephelometric turbidity units (NTU) or less. The product water was dechlorinated to avoid chloride burn of the vegetable leaves for the first three years of the field studies. Dechlorination was discontinued for the remaining two years with no detectable adverse impacts to the crops. The current California Department of Health Services Water Recycling Criteria for irrigation and other nonpotable uses are summarized in Table 1.

Three types of water were used during the field study: tertiary treated reclaimed water receiving the full treatment specified in the California water reuse criteria; tertiary treated reclaimed water receiving direct filtration; and local well water as a control. A 12-ha (30-ac) field site adjacent to the Castroville Wastewater Treatment Plant was divided into demonstration and experimental fields. Two 5-ha (12-ac) plots in the demonstration field were dedicated to irrigation using the direct filtration flow stream. Artichokes were grown on one plot and a succession of broccoli, cauliflower, lettuce, and celery was grown on the other plot. In the experimental field, artichokes were grown continuously from 1980 to 1985, and row crops were planted in rotation during the same time frame. A split plot design in the experimental fields allowed evaluation of water type and fertilization rate. Multiple replicates of each water type and each fertilization rate were evaluated in the experimental field, resulting in a total of 96 plots occupying 1.2 ha (3 ac). Separate irrigation systems were constructed to supply the three water types to the plots. Local farming practices were followed in both of the fields throughout the study.

Several baseline studies were carried out prior to the start of the five-year field demonstration phase of the MWRSA study to ascertain the uniformity of the soil on the site of the experimental plots and to assure the safety of down-wind areas from windblown aerosols during irrigation with reclaimed water.

The sampling program included the following:

• Sampling for metals and other chemicals included composite samples of each water type taken over a 3-day to 5-day period during each irrigation event.

• Grab samples were taken for other analyses, e.g., microorganisms and biochemical oxygen demand (BOD).

• Tail water resulting from furrow irrigation of row crops was analyzed for ten metals and sixteen chemical parameters.

• Surface soil samples were collected for bacteriological analyses within two days of irrigation, and soil profile samples were collected and analyzed for a variety of metal, chemical, and physical parameters. Samples also were collected and analyzed for cation exchange capacity, boron, and pH and salt content.

• Laboratory and field permeability tests were conducted. Edible and residual plant tissues were sampled and analyzed for bacteria, parasites, and metals, and edible portions of crops also were collected for metals analyses at each major harvest.

• Crop residues were sampled and analyzed for cadmium, zinc, and boron. Samples of edible tissue were taken from neighboring fields for bacteriological and metal analyses to provide comparative data for the study.

• The Castroville Wastewater Treatment Plant secondary effluent, tertiary treated effluent, well water used for irrigation, plant tissues, and soils were sampled for enteric viruses. The native virus concentration in the secondary effluent was low; therefore, virus-seeding studies using vaccine-grade poliovirus were carried out to estimate the virus removal efficiencies of the two pilot plants producing tertiary effluent.

• Groundwater in the demonstration fields was sampled for nitrate and other constituents.

In addition to the above, climatic conditions were continuously measured and recorded in order to aid in the evaluation of crop development and analyses, and a field study was performed to compare aerosols generated by spray irrigation with reclaimed water and well water.

Tertiary Treatment: Of the two tertiary treatment trains evaluated during the study, the more extensive train (i.e., secondary treatment, chemical coagulation, clarification, filtration, and disinfection) was somewhat more effective in removing or inactivating seeded virus than the direct filtration treatment train (i.e.,

secondary treatment, low dose coagulant/polymer addition, filtration, and disinfection), although both treatment trains reliably removed or inactivated more than 5 logs of virus during seeding experiments. Naturally occurring viruses were not detected in the product water of either tertiary treatment pilot plant.

Irrigation Water Quality: Both types of reclaimed water had higher levels of most chemicals, including metals, than the native local groundwater. Electrical conductivity, total dissolved solids (TDS), boron, chloride, and sodium in the reclaimed waters were similar to each other but higher than levels in well water. The TDS levels in all three types of water were below the severe problem range for irrigation water, and the sodium adsorption ratio in all three waters was in the favorable range for irrigation water.

Virus Removal/Inactivation: Measurable levels of viruses were detected in 53 of 67 samples (80 percent) of secondary effluent. No naturally occurring viruses were detected in disinfected tertiary effluent from either pilot treatment train throughout the study, and no viruses were detected in any of the crop or soil samples. An environmental chamber was constructed in a laboratory to determine virus survival under field conditions. The laboratory study indicated that the time required for 99 percent inactivation of seeded viruses ranged from 7.8 to 15.1 days, depending on the type of crop. Subsequent field studies produced similar results, and no seeded viruses were detected in any soil samples after 12 to 14 days of exposure.

Bacteria and Parasites: Coliform organisms were occasionally found in all three types of irrigation water. None of the samples taken from the three water sources or the soil indicated the presence of Salmonella, Shigella, Ascaris lumbricoides, Entamoeba histolytica, or other parasites. Parasites were detected in plant tissues during the first year of the study, but there were no differences between the levels in reclaimed and well water.

Aerosols: Aerosol tests were conducted during both daytime and nighttime irrigation. Microorganism transport via aerosols generated during spray irrigation of tertiary effluent was not significantly different from transport via aerosols generated during spray irrigation with well water, thus indicating that aerosol transmission of bacteria originating in the reclaimed water was unlikely.

Heavy Metals: There was no significant difference in any of the nine heavy metals studied (cadmium, chromium, cobalt, copper, iron, lead, manganese, nickel, and zinc) among plots irrigated with the different water types. Except for copper, the concentration of the metals did not show an increase in the soil during the five-year study period. Copper did increase gradually for all water types but remained below the average for California soils during the course of the study. Heavy metal input from commercial fertilizer impurities was far greater than from irrigation waters and accounted for the differences observed in soil samples throughout the five-year study period. Analyses of edible plant tissues indicated no consistent significant differences in heavy metal concentrations

between plants irrigated with reclaimed water and well water. Heavy metal concentrations in the three types of irrigation waters are shown in Table 2.

Crop Yield and Quality: Crop yield for most of the vegetables grown during the study was slightly higher for crops irrigated with either of the two reclaimed waters than with well water. It also was observed that increases in yield tended to level off at fertilizer applications below the commonly applied fertilizer application rate for the area, indicating that the typical full fertilization rate may be in excess of the crops’ requirements. Field crop quality assessments, shelf life measurements, and visual inspection did not reveal any difference between produce irrigated with reclaimed water and produce irrigated with well water. The use of reclaimed water to irrigate produce did not result in any increased spoilage over that encountered for irrigation with well water.

Marketability: A marketing firm was commissioned to conduct a study to determine the key issues associated with marketability of crops irrigated with reclaimed water. Interviews were conducted with individuals involved with produce distribution, such as wholesale-retail buyers, brokers, and store managers. Responses indicated that produce grown in reclaimed water would be accepted, labeling would not be necessary, and factual information would be useful to respond to customer inquiries that may arise. The major requirement of buyers was for produce to have a healthy appearance and be aesthetically attractive.

Respondents to the market study recommended that response to rumors that might occur regarding produce irrigated with reclaimed water should include clear, government-endorsed fact sheets and that support be given to developing an educational information program on the use of reclaimed water for crop irrigation.

Field Worker Health: The health status of each person assigned a field task during the field study was monitored through frequent questionnaires and initial and exit medical examinations. Neither questionnaire data nor medical examinations indicated any adverse health effects associated with working in fields irrigated with tertiary treated reclaimed water.

Based on the favorable results of the MWRSA study, a decision was made to design and construct a full-scale facility. Design of the treatment plant facilities, called the Salinas Valley Reclamation Project (SVRP), was completed in 1994 and was followed by design of the distribution system, which is known as the Castroville Seawater Intrusion Project (CSIP).

The SVRP includes the following facilities (design and regulatory criteria are summarized in Table 3):

• facilities to pump effluent from the existing secondary treatment plant to the new reclamation facility,

• rapid mixing of coagulant (alum) and flocculent chemicals (polymer or powered activated carbon),

• flocculation,

• dual media gravity filtration,

• disinfection using gaseous chlorine,

• diurnal flow equalization storage.

The CSIP distributes reclaimed water to 222 parcels of farmland within the 4,800 ha (12,000 ac) service area and includes the following:

• 74 kilometers (km) (46 miles [mi]) of reclaimed water transmission and distribution pipelines ranging in diameter from 0.2 meters (m) to 1.3 m (8 inches [in] to 51 in),

• 22 supplemental wells to augment reclaimed water flows at times of peak demand,

• 111 flow-metered turnouts for connection of irrigation piping by farmers,

• pressure, conductivity, and flow monitoring stations,

• a centralized control system,

• three booster pump stations,

• cathodic protection for ferrous metal piping.

Prior to startup an independent laboratory was hired to conduct a Reclaimed Water Food Safety Study, which continued after startup of the full-scale project. The primary objective of the study was to determine if any viable pathogenic organisms of concern to food safety, such as E. coli 0157:H7, Cyclospora, and Salmonella were present in reclaimed water produced by the SVRP. A secondary objective was to assess the ability of the treatment processes to remove or inacti-

vate pathogens that might be present in the influent wastewater. Samples were collected and analyzed during the 1997 to 1999 timeframe.

The study did not detect any E. coli 0157:H7, Cyclospora, Salmonella, helminth ova, viable Giardia, or culturable natural (in situ) viruses. Only an extremely low number of Cryptosporidium (in only two instances) was detected in any of the tertiary treated reclaimed water samples. Some of the study results are presented in Table 4.

The total capital cost of the Monterey County Water Recycling Projects was approximately $78 million. The total cost to treat and deliver reclaimed water to agricultural areas is estimated to be approximately $0.19/m³ ($225/acre-foot or $0.90/1,000 gallon) excluding secondary treatment costs but including both debt service from low interest loans and operation and maintenance costs for the two components (i.e., treatment facilities and distribution network) of the MCWRP. The 114,000-m³/d (30-mgd) wastewater reclamation plant was completed in 1997 and began delivering 76,000 m³/d (20 mgd) of reclaimed water for food crop irrigation in 1998. Reclaimed water is used to irrigate lettuce, celery, broccoli, cauliflower, artichokes, and strawberries. The service area is about 4,800 ha (12,000 ac).

Jaques, R.S. 1997. Twenty years in the making—now a reality: Nation’s largest project to provide recycled water for irrigation of vegetable crops begins operation. Proceedings of WEFTEC ’97 (October 18-22) 6:143-152.

Sheikh, B., R.P. Cort, W.R. Kirkpatrick, R.S. Jaques, and T. Asano. 1990. Monterey wastewater reclamation study for agriculture. Res. Jour. WPCF 62(3):216-226.

State of California. 2000. Water Recycling Criteria. California Code of Regulations, Title 22, Division 4, Chapter 3. Sacramento: California Department of Health Services.

U.S. Environmental Protection Agency. 1972. Water Quality Criteria 1972. EPA/R3/73/033. Washington, D.C.