Land Application of Wastewater in Arid Regions: the Challenge of Balancing Plant Water Requirements and Nitrogen Uptake

Walter H. Zachritz*, John Mexal**, and Ted Sammis**

Abstract

 

The application of municipal wastewater to land for treatment and disposal or “land farms” was one of the earliest forms of wastewater treatment technology. Land application systems have included application to edible and and non-edible crops, to rangelands, to forests and wood plantations, to recreational areas including parks and golf courses, and to disturbed lands such as mine spoil sites. Various designs of land application systems have been developed, including application of wastes to the soil-surface using Slow Rate (SR), Rapid Infiltration, and Overland Flow land treatment systems, and to the subsurface using leaching fields and absorption beds. The suitability of a particular system depends on site characteristics including soil properties, ground topography such as slope and relief, local hydrology, groundwater depth and quality, land use, climatic factors such as temperature, precipitation, evapotranspiration, wind, and length of growing season, and expected waste loading rates, as well as consideration of possible social and economic constraints. 

 

There has been a growing interest in using these systems in arid regions worldwide to supplement and reuse dwindling water resources.  Arid regions present complex challenges to the use of land application systems.  Many arid regions (Egypt and US/Mexico Border) are located in areas that lack infrastructure support and cannot afford expensive treatment technologies. For these regions; slow-rate, land application offers a low-cost, treatment unit that can be integrated with advanced, integrated ponds; facultative lagoons or other inexpensive primary and secondary treatment technologies. Properly designed land application units provide environmentally safe wastewater disposal by removing pathogens, nutrients, and suspended solids.  Additionally, the wastewater can be used to create value-added benefits such as wetlands, bosques; crops such as trees for fuel wood, pulp products, and lumber, cotton; and restoration of dryland desert ecosystems.  Critical to the design is the balance of seasonal, plant water requirements with plant uptake of nitrogen and the nitrogen and salt content of the wastewater. These factors must be carefully considered to assure system sustainability and minimize impacts to groundwater.  Cases in Ojinaga, Chihuahua, Mexico, Las Cruces, New Mexico USA, and Ismailia Egypt will be discussed.

Keywords: Wastewater reuse, land application, agriculture

 

*Executive Director of the Center for Arid Lands Environmental Management (CALEM), Desert Research Institute, Las Vegas, Nevada 89119 USA.

**Professor, Department of Agronomy and Horticulture, New Mexico State University, Las Cruces, New Mexico 88003 USA.

 

 

 

 

 

Introduction

 

Land application of wastewater is perhaps the oldest method for disposal and treatment of wastewaters.  Early systems were used in England as “Land Farms” that received untreated wastewater and night soil from nearby communities.  The dynamics of these systems were poorly understood and they were easily overloaded.  Most of these systems were abandoned and replaced by other technologies such as trickling filters.  In many areas of the world wastewater reuse has been practiced using a combination of treatment technologies that achieve a very high degree of treatment.  Many states in the western US have over the past twenty years been treating wastewater to tertiary treatment standards and then allow the wastewater to be reused for irrigation or for recharge to groundwater aquifers. While this is an effective method of treatment and reuse, it is very expensive and is rarely practiced in other regions of the world.  Land application systems that utilize the land as a treatment unit and not just as a disposal area are gaining acceptance in many arid regions.  These systems are cheaper to construct and can be operated by personnel with familiarity with common irrigation systems. 

 

With these systems, primary or secondary treated wastewater is applied to the land surface via furrow flood, micro-sprayer, or drip irrigation.  Biological oxygen demand (BOD), total suspended solids (TSS), and Fecal Coliform (FC) are partially removed in the primary or secondary treatment steps.  The land application unit removes additional BOD, TSS, and FC and nitrogen and phosphorus. The soil and plants (Watanabe, 1997) act as filters that trap and treat, through various mechanisms, contaminants in the wastewater and allow the remaining wastewater (effluent) to drain through the soil profile.  The wastewater provides an effective source of nutrients that the crop roots are able to assimilate.  The net effect is a beneficial system allowing for both the effective remediation of wastes and the recycling of water, nutrients, and carbon via biomass production (Kerr and Sopper, 1982; Bastian, 1986).  Not only is the waste problem managed, but also there is a potential for creating value-added products for economic development within the community from the resultant biomass.  Several options including tree plantations, bosque restoration, parkland, and crops have been proposed.  Crops such as alfalfa and cotton that are not intended for direct human consumption have been grown on wastewaters in many areas of the southwest US and northern Mexico. 

 

The basic approach to the design of land application systems is based on balancing the input of water and nitrogen as shown in the mass balance expression in Figure 1 as outlined by Metcalf and Eddy Inc (1990) and WCPF (1989).  The yearly wastewater application rate (Lw(p)) needed in the design of a wastewater irrigation system is based on the amount of total nitrogen (TN) allowed to enter the groundwater.  Typically this is given in terms of the concentration of NO3- -N in the wastewater reaching the ground water.  This is specified by regulatory agencies not to exceed the drinking water standard of 10 mg/L as N. The yearly water and nitrogen mass balance equations used to derive the design equation were outlined by) and are based on the mass balance equation shown in Figure 1. The nitrogen uptake by the crop is related to evapotranspiration and crop yield using the evapotranspiration production function. At this time this design model does not take into account loading from salt and interacts with the soil EC.

 

 

 

           

 

 

 

 

                                                             

                                                                 

Figure 1.  Mass Flow Diagram of the Land Application System with Inputs, Outputs, and Process Transformations.

 

 

The water balance for this system with a selected plant or crop can be determined from equation 1.  And the nitrogen balance is given in equation 2.  These equations must have locally derived data inputs and be solved to determine the area required for the land application system.

 

                                    Lw(p) = PET - P + Wp                                    (1)        

 

         

       Where:     

                        Lw(p)   =   wastewater hydraulic loading rate  (cm/mo);

                        PET      =   potential evapotranspiration rate (cm/mo);

                        P          =   precipitation rate (cm/mo); and

                        Wp       =   design percolation rate (cm/mo).

           

 

 

                                    Lw(n)  =  (Cp * (P - ET) + (U * 4.4))               (2)

                                                            ((1 - f)*Cn - Cp)   

              

               Where:

 

                                    Lw(n)     =          allowable hydraulic loading rate (cm/yr);

                                    ET        =          design PET rate (cm/yr);

                                    P          =          design precipitation rate (cm/yr);

                                    Cp        =          total nitrogen in percolating water (mg/L);

                                    Cn        =          total nitrogen in applied wastewater (mg/L);

                                    U         =          crop nitrogen uptake rate (kg/ha/year); and

                                    f           =          fraction of applied total nitrogen removed by

                                                            denitrification and volatilization

 

 

More importantly this information must incorporate plant nitrogen uptake rates and water use rates or crop coefficients to allow for proper balancing and ultimate system sizing.  Finally salt loading on the soil and its impact on plant survival and growth must also be considered.  This will determine plant selection.  Most plants are less tolerant to salt in the seedling stage than in more mature stages so alternative sources of water may have to be obtained.  Salt loading is not incorporated into the design equations but guidelines are provided in several resources such as WPCF (1990) and FAO (1992).  Systems we have dealt with have had salinities as high as 3,500 mg/L.  

 

In the following sections, we provide summary information about three land application systems we have been involved with in arid regions.

 

Ojinaga, Chihuahua, Mexico

 

Ojinaga is located in the state of Chihuahua, Mexico, and situated at the confluence of the Río Grande (Río Bravo) and Río Conchos on the U.S.-Mexico border.  The climate of this part of the Chihuahuan desert is arid, with an annual rainfall of less than 250mm.  The maximum recorded temperature is 50°C, with a minimum temperature of -10°C.  The combined population of Ojinaga with its sister city, Presidio, Texas, is approximately 27,000 inhabitants.  Ojinaga is demographically typical of smaller and intermediate-sized border communities.  However, the population of Ojinaga has dropped from about 26,000 in 1980 to 23,600 people in 1995 (USEPA, 1996).  Similarly, although Ojinaga is surrounded by approximately twelve thousand hectares of irrigable agricultural land, only half of this land is currently producing crops such as alfalfa, corn, wheat, cotton, melons, onions, and pecans.  One of the primary reasons that the land remains unfarmed is the landholdings average only five hectares each, making them too small for competitive agricultural production (Nuñez, 1997).  Further, salty irrigation water suggests soil salinization, thereby creating unsuitable conditions for the production of agronomic crops (Mexal, 1997). 

 

Historically, the municipal sewage has been piped directly into an anaerobic lagoon, which acts as a settling pond for separating the solids from the waste stream and providing some reduction in waste strength.  The 1.5 hectare (45,000 m3) unlined treatment lagoon, which had served the Ojinaga community for over 30 years, was taken off-line in 1994, because it had filled with settled solids.  A new 2-hectare (60,000 m3) anaerobic lagoon was constructed and put on-line in 1995, but it is expected to fill with collected solids within 6 years.  During the interim period between the shutdown of the old lagoon and the completion of the new lagoon (a period of several months), the untreated wastewater was discharged directly into the Río Grande.  Currently, about one-half of Ojinaga households are connected to the municipal wastewater system.  The municipal water authority in Ojinaga, Junta Municipal de Agua y Saneamiento (JMAS), hopes to have 95 percent of the households connected to the system within the next five years.  According to the president of JMAS, wastewater from Ojinaga is almost exclusively domestic in origin and current flow rates are expected to double when the entire community is connected to the system (Flores, 1994). The existing wastewater treatment system is a single cell anaerobic lagoon receiving both industrial and municipal wastewaters.  Industries in the area include bicycle assembly, slaughterhouse, and several cottage industries. These sources appear to contribute very little in the way of metals or toxic organics, but do contribute to hydraulic and organic loading of the wastewater.  The slaughterhouse wastewater is intermittent, but high strength.  The influent wastewater is treated by a manually cleaned bar screen to remove large solids.  Grit and materials that will pass the bar screen are collected in the anaerobic cell by simple sedimentation.  There is not active system for removal of these materials.  The single-stage anaerobic lagoon is designed to remove biological oxygen demand (BOD), total suspended solids (TSS), and fecal coliform (FC) to some degree. Wastewater from this system would flow to the land application unit for final treatment and disposal.

 

A pilot land application site (~1.1 ha total area) with 54 separate test plots about 0.02 ha in size each was established in April 1997.   Eucalyptus, Robinia, and Populus seedlings were transplanted using a 2m X 2m spacing.  All of the plots were manually flood irrigated with water pumped from an oxbow lake in order to allow for stand establishment.  The plots were flood irrigated with wastewater effluent and monitored for four years. Monitoring included wastewater effluent and groundwater water quality characteristics, soils, and plant growth (summary data in Table 1).  Local PET and rainfall data was used to determine optimum irrigation rates.

 

The results of this four-year study indicated that there was minimal overall impact to groundwater with no increases in fecal coliform or nitrate and about a 10 percent increase in salinity.  Tree growth results indicated that the optimal tree species were Eucalyptus and the 367 Hybrid Popular cultivar.  The growth rates achieved for these species exceed the expected results and may allow harvesting on five-year rotation basis.  This is one to four-years faster and than other studies of similar species and will improve the overall economic impact of the process. 

 

We estimate that a full-scale system for the community could support a land application system of about 500 ha.  We also estimated that the capital costs for this system could be almost 30 percent less than conventional treatment while producing a highly salable by product of wood pulp fibers.  This endeavor could produce community-based jobs and have a positive cash flow for the operation and maintenance.  Ojinaga has been involved in a master plan development process for the past three years for the development of a new treatment plant.  Despite our involvement with that process and the encouraging results for tree growth and pulp production it appears that the land application alternative will not be used for that site.   

 

Las Cruces, New Mexico USA

 

The City of Las Cruces West Mesa Industrial Park has a stand-alone, centralized system of wastewater treatment plant. The industrial park was started in 1982, but the centralized treatment facility was completed in the summer of 2000.  The goal of the system is to provide an effective method of wastewater collection and treatment that will encourage the expansion of the Park, and also help minimize the environmental impact of the Park on its surroundings by eliminating individual systems operated by Park tenants and providing a more attractive site for the location and investment of new industries. The primary gravity collection system within the industrial park consisted of 19,000 feet of 8” and 12” diameter collection pipes.  Because of the flat topography, some pipe trenches were in excess of 24 feet in depth.  These depths required special installation methods that included remote controlled construction equipment.

 

The aerated lagoons were identified as the best treatment technology to be utilized at the industrial park because they are reliable, have low capital and operations cost, and generate low sludge volumes.


The City of Las Cruces Pretreatment Ordinance requires that industrial and commercial wastes be treated to lower the levels of biochemical oxygen demand (BOD) and total suspended solids (TSS) to 250 mg/l and 200 mg/l, respectively.  This means that any loading variations should follow those of domestic wastewater and will be well within the treatment capabilities of the proposed facility.  The pretreatment requirement also insures that the facility will receive a fairly uniform influent quality. The proposed facility is expected to reduce these levels to produce an effluent quality of 30 mg/l BOD and 30 mg/l TSS.

 

The 1,514 m3/day system consists of the following treatment components:

 

·        channel comminutor and a manual bar screen,

·        horizontal flow grit channel for primary treatment,

·        influent flow recorder

·        a splitter box to divide flow between treatment trains,

·        two, 757 m3/day parallel train,  three-cell aerated lagoons with

·        a complete mix basin with a fixed floor fine bubble diffuser system,

·        a partial mix/settling basin with a fixed floor fine bubble diffuser system,

·        a holding pond to receive treated effluent, and

·        a pump station to lift treated effluent to a spray irrigation disposal site.

 

Lagoon systems historically produce low volumes of sludge. The sludge production for this facility at final design flow of 757 m3/day per process train is estimated to be 26,545 kg of sludge per year per train.  Five years of sludge accumulation in the lagoons would reduce the system capacity by approximately 15%.  Sludge will be removed from the lagoons every five years to maintain the most efficient use of the system capacity for treatment and will be disposed of at the City's sludge injection site.

 

One lift station and approximately 152 meter long force main transport the treated effluent to the land application site.  The land application site consists of 23 sprinkler zones, 1,600 sprinklers and 21,493 meters of 1.5” to 16” diameter pipe, covering 22.2 ha.  Wastewater is prefiltered and then micro spray irrigators are used to apply treated wastewaters to the site.  Construction of the land application site used low impact methods to maintain the native vegetation.  The application of the effluent wastewater is operated on a rotational basis to allow infiltration of the wastewater and the nitrogen uptake by the native plant.  The total capital costs for construction of the wastewater collection and treatment facilities was $2,700,000. The project was funded by the City of Las Cruces and USEPA.  The final storage pond was filled in the winter of 2000-2001 and effluent was initially applied to the land in early spring of 2001.  Soil will be monitored for the accumulation of salts and development of the vegetation cover.

 

Based on the water quality data there was concern about accumulation of salts and the high concentration of nitrogen in the wastewater.  Selecting native vegetation (primarily Fourwing Saltbush) allowed the system to accept high salt loadings while maintaining water application rates that would not have a negative impact on groundwater.  Ground water depths were about 75 meters for this site.  The goal of this system was to dispose of wastewater in a cost effective manner.  A secondary benefit was the creation of improved desert habitat. The city in this case was not interested in developing the site further although options such as a nine-hole golf course and production of mesquite as crop were discussed.

 

 

Ismailia, Egypt

 

The Ismailia Serrabium Wastewater Treatment Plant was built by Egypt and US AID in 1995 and is operated by the Suez Canal Authority. Ismailia has a population of about 500,000 and is located 9.4 km from treatment plant. A 1.2 km pipeline collects and supplies wastewater to the facility.  The treatment plant design capacity is 90,000 m3/d and the current flow is 80-85,000 m3/d.

 

A parshall flume measures inlet flows to the treatment plant.  Pretreatment for the facility consists of three self-cleaning bar screens and two grit chambers.  Flow is divided to two parallel treatment branches with three lagoons (aerated, facultative, and polishing lagoons) for each branch.  The aerated lagoon has 20 aerators (~30 hp each) installed with 10 operating at any one time.   The facultative lagoon has 10 aerators that used only as needed to increase dissolved oxygen. The polishing lagoon is 3.5 m deep to facilitate the removal of fecal coliforms bacteria and Ascaris eggs.  The total detention time of the system is 11 days, designed to reduce BOD, TSS, Ascaris eggs, and fecal coliforms bacteria. Ascaris eggs and fecal coliforms bacteria are considered the major health risk for using wastewater for reuse.  The treatment plant does not have a disinfection unit nor does it provide tertiary treatment for nutrient removal.  There are 10 installed monitoring wells around the site.  This treatment plant is a pretty standard design for Egypt and several other similar systems are located in other areas of the country.

 

The land application facility is about 2 years old and uses a land area of about 200 ha with up to 2,000 ha available.  The facility supports nursery and grow out operations.  About 75 ha have been planted since July of 1998.  All plants are planted on 3 m x 3 m spacing and drip irrigated with tertiary or secondary treated wastewater (90,000 m3/d) as shown in Figure 2. The wastewater treatment plant is located about 0.25 km away.  Wastewater is supplied the final sump station via an underground pipeline to the forest.  The wastewater is filtered through an inline screen and then through several sand filters before it is delivered to the irrigation system.  Ground water in the area is at about 8 m.

 

The nursery production capacity at Serrabium Forest is 100,000 trees per year.  The trees are grown in bags (12 x 15 cm), irrigated with wastewater, using a sand: clay: peat moss medium.  Tree species produced are P. pinea, P. halepensis, P. brutia var. eldarica, Khaya senegalensis, Cupressus arizonica, Cupressus sempervirens, Cupressus macrocarpa, Morus alba, Morus japonica, and Cassia. 

 

 

 

 

Figure 2.  Cypress (Cupressus) with Typical Drip Irrigation Layout.

 

The study for this site is scheduled to continue through 2002 with monitoring of water application rates, wastewater quality, and plant growth.  Some initial observations exemplify the need to optimize these systems.  At the Serrabium site we excavated a soil trench through the midline of an irrigated Italian Cypress (about 2 m in height) in very sandy (sugar sand) soil.  We observed that P. pinea (apparently a Dwarf provenance) had moist soil below 80 cm deep and about 1.5 m from the emitter.  We observed no soil layering or caliche development but attenuation of root development occurred at a depth of 40 cm or less.  Between plants in irrigated plots we observed very saturated soils just below the soil surface at the mid point between trees.  All this indicated a very high degree of over application of wastewaters to the tree plots with very little guidance using actual ET data. With this rate of application and a concentration of 45 mg/L or more of nitrogen in the applied wastewater and depth to groundwater of just over 5 meters there is a very high risk of contamination.  Egypt is planning to expand this model to at least a dozen other cities and without proper guidelines almost all these sites are at risk.

 

Summary

 

The data for each of the system discussed is provided in Table 1.  In all cases these are systems that are in arid regions with low rainfall and very high potential evapotranspiration.  Thus all water needs must be met by applied irrigation wastewater.  This water must be balanced against the needs of the plant and several other factors to assure the development of a sustainable design.  Salt and nitrogen provide significant antagonism in the development of the final system.  This must be resolved through proper

 

Table 1.  System and Wastewater Characteristics of the Three Land Application Processes.

 

Parameter/Site

Ojinaga, Mexico

Las Cruces, USA

Ismailia, Egypt

Treatment Train

Screens/Primary/Anaerobic                                                      Lagoon

Screens/Primary/Aerated Lagoon

Screens/Grit/Primary/Aerated /Facultative Lagoon

Disinfection

No

No

No

Effluent BOD, mg/L

29-43

30

46

Effluent TSS, mg/L

15

30

23

Effluent TN, mg/L

14-37

45

45

Effluent TDS, mg/L

1,950-2,220

2,000-3,000

550

Soil SAR, mg/L

5.5 – 7.0

N/A

N/A

Flow, m3/day

6,048

1,514

90,000

Land Application. Area, ha

500

22.3

200-2000

Rainfall, mm/year

280

216

<2.0

PET, mm/year

2,450

2,220

2,341

Selected Crop(s)

Eucalyptus/Hybrid Popular

Creosote Bush, FourWing Saltbush, Mesquite

Arizona Cypress, Afghan Pine, Eucalyptus, and others

Products

Wood Pulp Fibers

Habitat Creation

Wood Products and Fuel wood

 

plant selection, site characteristics considerations, and water quality. Proper safety precautions for personnel at these sites must be observed to minimize the risk of disease transmission. Finally the goals of the system in terms of the final product need to be incorporated into the process.  Systems can be developed that can produce revenue streams that can offset or some cases exceed operational costs. These systems can be public ally owned or private ventures, but infrastructure and community support must be developed well in advance and are essentially to the success of the project.

 

 

Selected References

 

Bastian, R.K., and J.A.Ryan. 1986. Design and management of successful land application systems. Pages 217-234 In  Proceedings. Utilization,treatment, and disposal of waste on land. Soil Science Society of America, Madison, Wisconsin.

Flores, J.C. 1994. Presidente, Junta Municipal de Agua y Saneamiento de Ojinaga. Personal Communication.

Foster, D.H., and R.S.Engelbrecht. 1973. Microbial hazards in disposing of wastewater on soil. Pages 247-270 In W.E.Sopper and L.T.Kardos (Eds). Recycling treated municipal wastewater and sludge through forest and cropland. The Pennsylvania State University Press: University Park, Pennsylvania.

Luecke, D.F. and C. De la Parra. 1994. From pollution to park. Experiment in Tijuana: a low‑tech approach to wastewater management. California Coast & Ocean. 10 (1): 7‑19

Metcalf & Eddy, Inc. 1991. Wastewater engineering: Treatment, disposal, and reuse. McGraw Hill: New York, New York. 815 pp.

Mexal, J. 1997. SCERP 1996 Quarterly progress report/Pilot study for an integrated waste treatment and disposal system along the US/Mexico border: Ojinaga community as a prototype. Unpublished.

Miller, R.W., and Donahue, R.L. 1990. Soils: An introduction to soils and plant growth. 6th ed. Prentice-Hall, Inc.: Englewood Cliffs, New Jersey. 768pp.

Nuñez, R. 1997. INIFAP agronomist. Personal communication, July, 1997.

Pescod, M. B.1992 Wastewater Treatment and Use in Agriculture.  Irrigation and Drainage Paper 47.  The Food and Agriculture Organization of the United Nations.

Samani, Z. A. and M. Pessarakli, 1986: Estimating Potential Crop Evapotranspiration with Minimum Data in Arizona, Transactions of the ASAE Vol. 29, No. 2, pp. 522-524.

Stewart, H.T.L., E.Allender, P.Sandell, and P.Kube. 1986. Irrigation of tree plantations with recycled water in Australia: Research developments and case studies. Pages 431-441 In Cole, D.W., C.L.Henry, and W.L.Nutter (Eds). The forest alternative for treatment and utilization of municipal and industrial wastes. University of Washington Press: Seattle, Washington.

U.S. Environmental Protection Agency. 1996. US-Mexico Border XXI Program Framework Document. EPA 160-R-96-003 and EPA 160-S-96-001. EPA: Washington, D.C..

Watanabe, M.E. 1997. Phytoremediation on the brink of commercialization. Environmental Science and Technology. 31 (4): 182-186.

Water Pollution Control Federation (WPCF). 1990. Natural systems for wastewater treatment. Water PollutionControl Federation, Alexandria, Virginia. 270pp.