By Jim Jordahl, Henriette Emond, and Mark Madison
Summary: With the WEFTEC show slated for the middle of this month, water reuse is bound to be a topic of discussion at the Los Angeles exhibition. In fact, this article is excerpted from a presentation that will be given at the event.
The Acoma Pueblo, N.M., sewage treatment, storage and evaporation lagoons were unable to adequately handle existing sewage flows, and a more permanent solution to wastewater disposal was needed. Ongoing construction of housing developments and community buildings were expected to further overload the existing lagoon system. Land application of the wastewater to a nearby field was identified as an acceptable alternative for reusing the wastewater and reducing the volume of water that must be evaporated from the lagoons.
The objective of this project was to design a low-cost, low-maintenance reuse system to handle sewage flows from the existing community and expected growth. A long-term goal was to transition the crop to which the reuse water was applied from saltgrass to cottonwood trees as a potential source of fuel.
The quality of the wastewater accumulated in the lagoons posed a significant challenge for implementation of a reuse system. The wastewater was somewhat saline with an electrical conductivity of 2.7 deciSiemens per meter (dS/m), pH was extremely high at 9.8, and the sodium adsorption ratio (SAR)—the amount of sodium relative to the amount of calcium and magnesium—was also extremely high at 11.8. Very few plants can tolerate application of water with these properties, especially through a sprinkler system.
Water high in sodium is toxic to most plants if applied directly to the leaves. Over time, the water quality for reuse is expected to improve as the water chemistry becomes more reflective of the influent. The residence time of the wastewater in the lagoons will decrease, evaporation will be less, and the reuse water will become somewhat less concentrated. The projected flows are presented in Table 1. Wastewater flows were expected to increase as a number of additional housing units and community buildings came online.
Data from recent water quality samples are presented in Table 2. Raw sewage flows into Lagoon 1 that provides treatment primarily by settling the heaviest solids. The wastewater then progresses through Lagoons 2, 3 and 4, which provide detention time for biological treatment. The water quality in Lagoon 3 on the west side of the creek is considered to be representative of what the irrigation water quality will become over the “long-term.” The “short-term” water quality is the chemistry of the water that has accumulated in Lagoon 4. This was the first water to be applied to the reuse site.
The effluent distribution system consists of the lagoons, pump station, filtration system, chlorination system, conveyance mainline, field sub-main, and the sprinkler irrigation system. From the force mainline, effluent is transmitted by a buried PVC sub-main to HDPE tubing laid on the field surface. The application system utilizes micro-spray sprinkler heads mounted on stakes and connected to the tubing with a small hose and barb.
The pump station with associated suction piping, filtration system, hypochlorite generation unit and interconnecting piping are housed in an equipment building consisting of a concrete slab with a wood structure. The structure also includes an exhaust fan, lighting and other miscellaneous electrical systems.
A series of six lagoons are used; four to treat and two others to evaporate wastewater. The wastewater reuse system withdraws water from Lagoon 4. A pre-fabricated pump station was built on the berm of one of the six lagoons. The pump station is Flowtronex PSI Inc.’s horizontal, centrifugal, fixed-speed prefabricated pump station. The pump system has a 30-horsepower motor with a pump designed to operate at 350 gallons per minute (gpm) at 70 pounds per square inch (psi).
The filtration system is located adjacent to the reuse pump station. The filter unit is a Netafim 3-inch by 5-unit Disk Kleen, 80-mesh filter battery with stainless steel manifolds, and a Netafim eight-station backflush controller. Filter backflush reject water is directed to one of the evaporation lagoons through a two-inch diameter pipeline.
The tablet chlorination system consists of a Hammonds unit that chlorinates the water by slow dissolution of calcium hypochlorite tablets. Water is taken from downstream of the filter system, fed through the chlorination system, and then injected in the pipeline between the pump station and filter system.
The pipeline consists of approximately 3,800 feet of 8-inch mainline, 600 feet of 6-inch, 800 feet of 4-inch, and 300 feet of 2-inch. The 8-inch section includes a section of directional drilled HDPE installed under the Rio San Jose, an irrigation canal “undercrossing,” and a bore and jack section under the roadway. To prevent cutting the roadway and disrupting traffic, a hole was bored under the highway and a steel pipe was jacked through the hole to provide a casing for the mainline to pass through. The canal and road undercrossings include 12-inch casings. Other items incidental to the pipeline include isolation valves, air/vacuum release valves, drain valves, trench work and other miscellaneous items.
The irrigation system layout was designed for a 24‑by-24‑foot grid for sprinkler spacing. Each sprinkler head has a 0.35‑gpm flow-control nozzle. The sprinklers closest to the edges of the fields have partial circle heads to prevent over-irrigation beyond the planting area. Heads in adjacent spray lines are offset to form a diamond-shaped pattern. The irrigation application rate is about 0.06 inch/hour and less than the soil infiltration rate, which was 0.16 to 1.45 inches per hour for subsoils at the site. If the site is ultimately converted to grow trees, the spray lines will be in line with the trees in every other tree row. The microsprinkler nozzles will be spaced with one tree between each nozzle.
Reclamation of the site and ongoing operations require careful monitoring and data interpretation. Required monitoring includes quantities and flows at the pump station. Piezometers—small-diameter wells—are used to monitor for a perched water table (zone in which soil is saturated with water well above the main water table), soil moisture sensors are used to adjust irrigation rates, and suction lysimeters—devices used to extract water from unsaturated soils—are used to monitor soil pore water quality below the root zone.
Piezometers are checked regularly for the presence of water during irrigation to make sure a perched water table isn’t being created. Any water in the piezometers indicates that a perched water table may be forming. The soil moisture sensors and data loggers are regularly checked to make sure that irrigation rates are appropriate. Key indicators of an appropriate irrigation rate are adequate moisture for plant growth and controlled leaching without the creation of a perched water table or water ponding on the ground surface.
The chemistry of both the irrigation water and the soil have to be considered together to develop the best possible plan for site reclamation and establishment. Samples of the irrigation water will be tested regularly in helping plan adjustments to operations and soil amendments. Parameters to be tested include: pH, NO3 + NO2-N, Total Kjeldahl Nitrogen (TKN), Na, Cl, TDS, electrical conductivity (EC), NH3, total alkalinity, HCO3, CO3, SO4, Ca, B, and Mg.
Taking a sample
At least once a year, composite soil samples are collected to determine the progress of the reclamation process and adjust amendment application rates. Analytical methods are those recommended by New Mexico State University. Parameters to be tested include: pH, Mn, Fe, Cu, Zn, electrical conductivity of the soil saturation extract (ECe), SARe (water extract Ca, Mg, Na), CaCO3, P, K, and B.
Soil pore water samples from the suction lysimeters are analyzed to help monitor the progress of the reclamation process. Parameters to be tested include: pH, NO3 + NO2-N, TKN, Na, Cl, TDS, EC, NH3, total alkalinity, HCO3, CO3, SO4, Ca, B, and Mg. Data from the monitoring effort are analyzed and synthesized on a regular basis (weekly and monthly, depending on the parameter). This information is used to recommend adjustments to the operations necessary to maintain progress toward reclamation of the site.
Additional monitoring is performed as an overall assessment of the system performance and to ensure the saltgrass or tree system is healthy, and that nutrient concentrations in the soil are within acceptable limits. A regular visual inspection of the vegetation is conducted to ensure the health of the plantation and identify any potential problems with the vegetation.
Leaf tissue samples are collected annually during a period of rapid growth to assess the fertility status of the plants. Samples are analyzed for the following parameters: P, K, N, S, Mg, Ca, Na, Fe, Mn, Cu, Zn, and B. Table 3 summarizes the major parameters monitored at the reuse site.
Implementation of a saltgrass-based land application system has been successful for Pueblo of Acoma. For relatively low cost, it has provided the needed additional capacity for the overall wastewater treatment system. Saltgrass provides a temporary or permanent reuse alternative for similar communities in the Southwest. Saltgrass is relatively easy to establish, and is highly tolerant of both salinity and high sodium, even if wastewater is applied directly to the foliage.
The authors would like to thank the Pueblo of Acoma and the Indian Health Service for their assistance on this project.
- “Skyline Lagoon Wastewater Reuse Site Operations Plan: Final Report,” CH2M Hill, prepared for Pueblo of Acoma Housing Authority, Pueblo of Acoma, New Mexico, March 2002.
- “Wastewater Reuse System Conceptual Design: Final Report,” CH2M Hill, prepared for Pueblo of Acoma, January 2001.
- McBride, M., “Environmental Chemistry of Soils,” Oxford University Press, New York, 1994.
- Rhoades, J.D., and J. Loveday, “Salinity in Irrigated Agriculture,” pp. 1089-1142, In Irrigation of Agricultural Crops – Agronomy Monograph, No. 30, SSSA, Madison, Wis., 1990.
- U.S. Department of Agriculture Soil Survey Division Staff, “Soil Survey Manual,” USDA Handbook 18, Washington, D.C., 1993.
- USDA/NRCS, “Soil Survey of Cibola Area, New Mexico, Parts of Cibola, McKinley, and Valencia Counties,” U.S. Government Printing Office, Washington, D.C., 1993.
- USDA Salinity Laboratory Staff, “Diagnosis and Improvements of Saline and Alkaline Soils,” Agriculture Handbook 60, USDA, Washington, D.C., 1954.
About the authors
Jim Jordahl, Ph.D., is an environmental scientist and senior technologist with CH2M Hill, of Des Moines, Iowa. He can be reached at email: email@example.com. Henriette Emond, Ph.D., P.E., is an agricultural engineer with CH2M Hill in Portland, Ore., and Mark Madison, P.E., is an agricultural engineer and principal technologist with CH2M Hill in Portland,Ore.