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Title: WRRCTR No.111 Recycling of Sewage Effluent by Sugarcane Irrigation: A Dilution Study, October 1976 to June 1977, Phase II-A 
Author: Lau, L. Stephen; Ekern, Paul C.; Loh, Philip C.S.; Young, Reginald H.F.; Dugan, Gordon L.
Date: 1977-08
Publisher: Water Resources Research Center, University of Hawaii at Manoa
Citation: Lau LS, Ekern PC, Loh PCS, Young RHF, Dugan GL. 1977. Recycling of sewage effluent by sugarcane irrigation: a dilution study, October 1976 to June 1977, phase II-A. Honolulu (HI): Water Resources Research Center, University of Hawaii at Manoa. WRRC technical report, 111.
Abstract: Phase II-A is a project that is an extension of a recently completed 4-yr project (July 1975) entitled "Recycling of Sewage Effluent by Irrigation: A Field Study on Oahu," hereinafter referred to as Phase I. The main purpose of Phase I was to apply secondarily-treated sewage effluent from the Mililani STP to thirty 0.04-ha (O.1-acre) test plots of sugarcane located in Oahu Sugar Company's (OSC) Field No. 246 in central Oahu, Hawaii. The results of this study showed that sewage effluent applied for irrigation during the first year and Waiahole Ditch water the second year of the 2-yr sugarcane crop cycle increased the sugar yield by 6% over control plots, whereas, sewage effluent applied during the full 2-yr cycle reduced the sugar yield by 6% below the control plots. Sewage-borne viruses and
coliform bacteria in the applied sewage effluent were inactivated within the shallow root zone; and salts, including nitrogen, did not appear to leach below the root zone in any greater quantities than that of the companion ditch-irrigated plots.
The results of Phase I appear promising in terms of increased sugar yield, an additional irrigation water source, and alleviation of a sewage effluent disposal problem; however, it is not presently considered economically
feasible to construct and maintain a separate ditch water and sewage effluent field distribution system, thus, the question arises as to the optimum dilution of sewage effluent with ditch water for a single field
distribution system so that sugar yield will not be decreased and hopefully increased over present plantation practices .
The Phase II-A portion of the project attempts to determine the dilution
necessary for an optimal balance of water disposal and sugar yield.
Secondary objectives of the project are to monitor the quantities of nitrogen
leaching past the root zone and continued monitoring of sewage effluent for the presence of human enteric viruses. The reporting period for this progress report is from October 1976, when the sugarcane was planted in the test plots, to June 1977. The same 30 test plots and sugar cane variety (Hawaiian variety 593775) as used in Phase I were incorporated in Phase II-A. Five irrigation treatments for the 2-yr cycle with six replicates in a randomized block design were: (1) ditch water, (2) 12.5%, (3) 25 %, (4) 50% effluent diluted with ditch water, and (5) effluent the first year and ditch water the second year. Irrigation rounds of up to 10 cm (4 in.) were applied bi-weekly, and tensiometers in selected test plots monitored water stress conditions. Representative soil samples were collected and analyzed shortly after sugarcane planting. Commercial fertilizer applications of N, P, and K to the test plots were completed in June 1977.
Four crop logs were made on sugarcane growth in the test plots. The physical appearance of that growth was very good; however, the relatively low K-H2O index and the high total sugars from the crop log indicate a possible growth imbalance. Rainfall during this period, due to a very dry winter, was nearly one-half of normal, whereas, evaporation was more than 50% above normal.
Secondary effluent from the Mililani STP is typical of domestic sewage, although it has a higher BODs than experienced during Phase I. The effluent is monitored on a routine basis at the STP, and also when the test plots are irrigated. Soil leachates are collected during each irrigation from representative plots through 5-cm (2-in.) diameter by 1.52-m (5-ft) long porous plastic "Porvic" samplers placed horizontally in the subsoil beneath the root zone. The concentration of nitrogen in the leachates collected beneath the root zone fluctuates with fertilizer applications, but after fertilizer applications were completed, increases with the higher percentage of effluent in the irrigation water were observed.
Series/Report No.: WRRC Technical Report
Sponsorship: Board of Water Supply and Department of Public Works, City and County of Honolulu
Pages/Duration: viii + 52 pages
LC Subject Headings: Water reuse -- Hawaii -- Oahu.
Sewage irrigation -- Hawaii -- Oahu.
Sugarcane -- Irrigation -- Hawaii -- Oahu.
Microirrigation -- Hawaii -- Oahu.

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