Trickle Irrigation Using Waste Water Affluent - A Case study
Problem Based Learning

Introduction

Problem Submitted to Trickle-L, a list server  dedicated to solving trickle irrigation problems

A.farmer has the option to take tertiary treated effluent instead of potable water. The existing system  rarely gets much maintenance. The filters are located at each block (7-9 acres each) and are only 50 mesh screens. The overall system is comprised of 36- 3" valves covering 360 acres. The crop is pistachios. The existing emitters are multi-outlet, continuous flushing, stacked silicon orifice plate type. There are lateral end caps for flushing, but this is rarely ever done.  The water provider chlorinates the water leaving the plant, on the order of 6-7 ppm Total Chlorine. The residual chlorine at the farm's point of connection will only be 1-3 ppm, since the pipeline is very large, and quite a long distance of conveyance that uses some of the chlorine to oxidize the material in the conveyance line.  The municipality assured the owner that the pressures will remain in the 55-60 psi range, with only very brief interruptions in availability. .

Question

Will conversion cause emitter plugging or slimming due to a lack of maintenance or can the system operated at before using potable water. The farmer considers any additional labor to maintain the irrigation system to be a problem.The farmer does not receive any adjustment in the cost of water. 

Should the farmer require the municipality to treat the water (chlorine, acid, gypsum, etc.)  since the benefit of making the conversion flows mostly to the municipality.

Solution

Stacked orifice plates represent  a reasonable emitter for use of wastewater. Other emitters that are used for waste water are emitters impregnated with a herbicide to impede root growth into the emitter.  To test the water for plugging problems use  the  jar test with some of the effluent to see what settles out. Add a little 10-34-0 fertilizer  as well and see if precipitates are a problem, and request the treatment plant records for % suspended solids in the
effluent. If the water has been even partly "polished" and subsequently chlorinated  then it will probably pass the "jar test" without a problem and  the  present drip system  design is sufficient for use with waste water. However, watch the sodicity of the water. Sodium is not a problem with pistachios, but if  the SAR is > 5x EC then  the water  may  change  the soil structure/infiltration. If the is the case then gypsum should be added to the water by the water treatment plant to maintain a good SAR.