Tertiary treatment for water re-use on food crops

A new UV treatment system in Santa Barbara treats wastewater, providing effluent disinfection to tertiary levels: the water is now to be reused for spray irrigation on edible food crops

Like many municipalities, Santa Barbara County in California has struggled in the face of arid conditions with increased demand for fresh water to meet the needs its urban population, agriculture and industry.

The Laguna County Sanitation District is helping to stretch the county's water supply by implementing a new UV treatment system to treat wastewater for reuse.

The upgraded system will disinfect effluent to tertiary levels, allowing the water to be reused for spray irrigation on edible food crops.

The Laguna County plant was built in 1959 and treats municipal and commercial wastewater from the unincorporated community of Orcutt and portions of southern Santa Maria.

The plant was originally designed to treat wastewater to secondary levels (removal of suspended solids and biofiltration).

Wastewater which undergoes secondary treatment is approved for release into nearby bodies of water and for irrigating areas where the recycled water will not come into direct contact with edible portions of food crops (ie, it can be used to water the roots of apple trees but not sprayed directly on the fruit).

The recycled water was used first to irrigate sugar beet crops and more recently to irrigate pasture land for cattle grazing.

However, changing circumstances and water regulations caused the county to re-evaluate its wastewater treatment operations.

"In the late 1990s, we started to run into problems with our disposal capacity and meeting effluent guidelines," explained Laguna County Civil Engineer Manager, Martin Wilder.

The facility had a treatment capacity of 3.2 million gallons per day (mgd), but the Central Coast Regional Water Quality Control Board restricted the facility to an effluent flow of only 2.4 mgd, due to limited disposal capacity.

Additionally, residual salts from residential water softeners was pushing the concentration of total dissolved solids (TDS), sodium and chloride, just above the discharge requirements set for the county.

"We looked at several options including expanding our disposal capacity for the secondary effluent we were producing," said Wilder, "but in the end we decided to upgrade treatment to tertiary levels in order to use the recycled water for a greater number of possible applications".

The District engaged the services of the environmental engineering firm CH2M Hill to design a new treatment facility which would increase the treatment capacity, eliminate the problem with dissolved solids and produce effluent treated to tertiary levels.

The ability to classify recycled water as being treated to tertiary levels is dependant upon meeting the stringent guidelines set forth in California's Code of Regulations - Title 22, Water Recycling Criteria.

The system chosen would need to incorporate both filtration and disinfection to meet these guidelines.

Membrane technology is for filtration of influent and a reverse osmosis system is used to treat 0.5 mgd of high TDS influent.

For disinfection, Ultra Violet (UV) technology was chosen.

"We knew that we didn't want a chlorine-based system because of the high operating cost and potential hazards in storing and handling the needed chemicals," explained Wilder: "Our consultants at CH2M Hill recommended UV as an alternative.

We really liked the comparatively low operating and maintenance costs in addition to the fact that UV is safe and leaves no chemicals or other by-products in the treated water".

UV is the part of the electromagnetic spectrum responsible for the bacterial effect of sunlight.

In UV treatment systems, that effect is artificially recreated using UV lamps.

UV typically achieves a 99.9% kill-rate in most common micro-organisms.

The UV is absorbed by the cellular DNA, altering the DNA and rendering the micro-organism incapable of reproduction.

UV has been established for over 70 years as a reliable means of disinfecting process water, and today is widely used for food and beverage production, pool disinfection, wastewater treatment and pharmaceutical manufacturing.

The District and CH2M Hill evaluated several UV treatment systems, including open and closed channel designs, and chose Inline units from Aquionics.

The Inline units were selected based on their closed channel design, which Wilder noted would be easier and less expensive for the District to operate compared to an open channel 'trough' design.

There are several advantages of a closed channel design which make it easier and less costly to operate.

An important consideration is the low head loss; the number of lamps and baffles required in an open channel system cause extremely high head loss.

Additionally, the closed channel design eliminates the opportunity for photo-reactivation by delivering a lethal medium pressure UV dose and then keeping the treated water away from light, which may enable DNA to repair itself.

Finally, in a closed channel system, there is no operator contact with wastewater effluent while changing lamps.

The lamps slide into quartz sleeves contained in the treatment chamber.

Another consideration for Wilder was that the compact chambers install directly into existing pipework with a significantly smaller footprint than open channel configurations.

At the new facility, after being filtered through the membrane and reverse osmosis systems, the two influent streams (normal TDS and low TDS streams) are combined for disinfection.

The water then flows through four (three operational and one standby) Aquionics Inline units arranged in series.

The series arrangement was used in order to deliver the correct UV dose needed to meet the Title 22 disinfection requirements given the flow rate and quality of the incoming water.

The Aquionics Inline units use high-output medium pressure UV lamps to deliver a high dose of UV disinfection with fewer and smaller lamps than comparable low pressure systems.

Fewer lamps also translate to less parts requiring maintenance.

UV control equipment includes direct UV monitoring of actual dose, flow meter, online transmittance monitoring and variable power control.

Self-cleaning quartz sleeves keep the lamps free of organic deposits from the water, resulting in more consistent system performance with very little maintenance.

The system was installed by Cushman Contracting and began operating in April of 2003.

The District continues to work with CH2M Hill on the validation testing and reporting phases required by the Title 22 Water Recycling Criteria and Ultraviolet Disinfection Guidelines for Drinking Water and Water Reuse published by the National Water Research Institute (NWRI) and the American Water Works Association Research Foundation (AWWARF).

The NWRI/AWWARF guidelines pertain specifically to UV disinfection systems and specify system performance goals and validation testing protocols.

The UV facility must receive approval from the California Department of Health Services before the treated water can be distributed as reuse water to end users.

According to Sylvester Hsu, Task Manager from CH2M Hill on the UV Validation Testing and Reporting, initial validation testing has indicated that the "results so far look quite favourable".

The UV disinfection has resulted in effluent with bacterial levels below those required by the Title 22 criteria which is 2.2 MPN per 100mL for total coliform.

"The validation has also shown that MS-2, used as a surrogate organism in the testing phase, experienced 3 to 4 log removal," Hsu elaborated: "UV dosages delivered by the system have also been higher than that require under the NWRI/AWWARF guidelines".

"Our primary goal was to treat effluent to within the limits of the Title 22 guidelines; based on the tests we've done so far, we've achieved that," agreed Wilder.

The Laguna County Sanitation District received approval from CDHS on October 18, 2005 for a tertiary treatment system producing recycled water meeting California's Code of Regulations - Title 22, Water Recycling Criteria.

Their customers will include strawberry and broccoli growers, golf courses and cattle ranchers.

They are also investigating potential industrial uses for the treated water.

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