WRRCTR No.135 Post-Secondary Treatment of Effluent for Drip Irrigation: July 1977 to March 1980, Phase II-B

Lau, L. Stephen
Young, Reginald H.F.
Young, George P.
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Water Resources Research Center, University of Hawaii at Manoa
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A pilot post treatment system was designed, installed, and operated at the Mililani Sewage Treatment Plant (STP), O'ahu, Hawaii, to study the necessary treatment of activated sludge effluent for drip irrigation according to prevailing Hawaiian practice. Principal unit processes were filtration with horizontal pressure anthracite and sand filters, and treatment with chlorine and other chemicals producing equivalent chlorine dosages. Initial operation with 100 and 50% secondary effluent diluted with Waiāhole Ditch water resulted in drip-emitter plugging exceeding the ≤ 1O% criterion, as did subsequent operation with crushed granite replacing the sand media and processing 100% effluent. The filter design and configuration was inadequate to cope with either the 100 or 50% effluent diluted with water, and appeared to be the cause of failure of the post treatment system. The post treatment system was then modified with vertical sand filters processing 25% secondary effluent diluted with ditch water. Chemical treatment consisted of chlorination during backwash cycles. For a 2-mo period, this modified system maintained emitter plugging at 10%, with irrigation pressures as low as 11.032 x 10^4 Pa gage (16 psi), and backwash triggering pressure differentials as high as 68 950 Pa gage (10 psi). Backwash water usage at the latter level was as low as 8%. However, subsequent operations during a period noted to correspond to major changes in Mililani STP operations yielded abnormal system performance, with plugging recorded between 40 to 50% while the STP effluent suspended solids were as high as 141 mg/l. Rejuvenation of the drip tubing was only gradually achieved using various remedial measures. The entire post treatment operation was abbreviated and terminated after 3 months because of the lack of ditch water.
effluent reuse, hydraulic design, irrigation systems, waste water treatment, waste water use, Hawaii, drip irrigation, dual-chambered tubing, single-chambered tubing, Mililani, Oahu
Lau LS, Young RHF, Young GP. 1981. Post-secondary treatment of effluent for drip irrigation: July 1977 to March 1980, phase II-B. Honolulu (HI): Water Resources Research Center, University of Hawaii at Manoa. WRRC technical report, 135.
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