ADVERTISEMENTS:
In this article we will discuss about the agronomic and public health assessment of reclaimed water quality for landscape irrigation.
Rafael Mujeriego, Maria Carbo
Department of Hydraulic, Coastal and Environment Engineering Universidad Politécnica de Cataluña Gran Capitán, D-l, 08034 Barcelona, Spain
ADVERTISEMENTS:
Lluis Sala
Consorci de la Costa Brava, Placa Josep Pla 4, 17001 Girona, Spain
Josep Turet
Estudis Universitaris de Vic, Miramarges 4, 08500 Vic, Spain
ADVERTISEMENTS:
Abstract:
The Mas Nou golf course has been using disinfected secondary effluent for irrigation since September 1989. Although adjusted-SAR values and EC of irrigation water have generally fallen in the non-restricted irrigation categories, EC values were initially higher than desirable for turf irrigation, due to high effluent chloride concentration.
A definite solution was reached in the summer of 1993 with completion of a new surface water transfer. Since 1993, EC values in summer effluent range from 1.2 to 1.5 dS/m. A new water and storage pond management system has been proposed to optimize both water and nutrients application, using only the most adequate water source available at a given time.
The relatively low iron contributions of reclaimed water have resulted in occasional spots of ferric chlorisis, which have been recovered with localized application of iron compounds. Fertilizer savings have ranged from 0.06 to 0.12 US $/m3 of reclaimed effluent. Weekly analyses of reclaimed water show that faecal coliforms and faecal streptococci concentrations lower than 100 cfu/100 mL can be consistently achieved.
By mechanical mixing of top oxygen-rich waters with bottom oxygen-depleted waters in the storage ponds, odour problems have been totally eliminated. Close collaboration with the green keeper has resulted in a more favourable attitude to the water quality requirements of reclaimed effluent and a more effective and economical fertilization program. The green keeper perception has gradually evolved from a passive acceptance of the requirements of using reclaimed water to a positive recognition of its benefits.
Introduction:
The continuous increase in water demands in many semi-arid areas of the world, due to permanent and seasonal population growth, together with the severe and prolonged droughts experienced in some of those areas over the last decades, and the increasing restrictions on water quality levels for receiving waters have made clear the limitations of present and planned conventional water supplies to satisfy current and expected potable water demands.
The coastal resort area of Costa Brava, located in the northern Mediterranean coast of Spain, illustrates quite well the problems and alternatives available for water resources management under limited conventional water supplies, important flow reductions due to seasonal droughts, an eight-fold increase in water demands due to summer tourist populations, and increasing requirements for aesthetic and public health quality of inland and coastal waters.
Landscape irrigation and particularly golf course irrigation represent a growing water use, due to the tradition of public and private gardens and the significant role of golf courses as an added amenity to tourists from central and northern Europe.
In this scenario the Consorci de la Costa Brava (CCB), the regional water supply and sanitation agency, launched in 1991 a two-year demonstration project of a planned water reuse project, in collaboration with the Junta de Sanejament (Catalonia’s Sanitation Agency), the Universidad Politecnica de Cataluña (UPC), and the Mas Nou golf course.
ADVERTISEMENTS:
The objective of this demonstration project was to develop technical and management criteria for reclamation of secondary effluent and irrigation of a golf course whose main water source is the reclaimed effluent provided by CCB. The Mas Nou golf course demonstration project was subsequently extended under the joint sponsorship of CCB and UPC, and the results obtained are presented in the following sections.
The Mas Nou golf course has been using disinfected secondary effluent for irrigation since September 1989. Secondary effluent from the Castell-Platja d’ Aro wastewater treatment plant (WWTP) is disinfected in the 2,200-m pipeline used to bring reclaimed water up to the 300 m elevation of the golf course; the disinfectant used is sodium hypochlorite (15% Cl2), with doses ranging from 25 to 45 mg Cl2/L depending mainly on effluent’s turbidity and/or ammonia nitrogen concentration.
Reclaimed effluent is stored in two landscape ponds connected in series, each one used for irrigating a different area of the golf course. The reclaimed effluent flows into Pond 1, which has a capacity of 13,300 m3, a maximum depth of 4.5 m and serves to irrigate 21 ha of turf; water from Pond 1 flows into Pond 2 which has a capacity of 21,000 m3, a maximum depth of 5.5 m and serves to irrigate 13 ha of turf. Secondary effluent and water from both ponds have been intensely monitored for agronomical and microbiological quality since 1989.
Castell-Platja d’ Aro WWTP:
The treatment system of Castell-Platja d’ Aro WWTP is a conventional activated sludge process, and was designed to treat the combined sewer flows of three nearby resort towns. The residential and tourist character of the area results in marked flow variations between winter (lower than 10 000 m3/day) and summer (higher than 30 000 m3/day). However, operation and maintenance by skilled and motivated personnel results in an effluent of consistently good quality all year round. Table 1 shows the effluent quality obtained during 1994.
Reclamation of secondary effluent has been limited to disinfection with sodium hypochlorite, using the 60-min contact time plug flow reactor provided by the pipeline needed to bring reclaimed water to the first landscape pond of the golf course.
Daily flows of reclaimed effluent used by Mas Nou golf course vary with irrigation water demand, reaching its maximum value (slightly over 2,000 m3/day) in July and August. As Table 2 indicates, the volume of reclaimed water used annually for irrigation has ranged from 152,000 m3 in 1994, when the facilities where practically closed until July, and 280,000 m3 during the particularly dry years of 1991 and 1993.
Agronomic Assessment:
Water Salinity:
ADVERTISEMENTS:
Water salinity has been measured by electrical conductivity (EC), and specific determinations of chlorides, sodium, potassium, calcium and magnesium. Between 1989 and 1991, the EC of summer effluent was considered unfavourable for turf irrigation because of its relatively high values: from 2 to 4 dS/m, with occasional values of up to 7 dS/m.
Although adjusted-SAR values and EC have generally fallen in non- restricted irrigation categories, according to the criteria developed for the preservation of soil permeability, EC values were higher than desirable for turf irrigation, due to high effluent chloride concentration.
During the severe drought of 1989-91, high EC values were observed partly due to excessive water supply withdrawals, which favoured marine intrusion into the nearby coastal aquifer. The higher rainfall measured during the fall of 1991 and all through 1992 resulted in a significant reduction of the EC of potable water.
A definite solution to the problems caused by high EC values both in potable water and secondary effluent was reached in the summer of 1993 with completion of a new surface water transfer from El Pasteral reservoir – 50 km inland – as an additional water supply for the Costa Brava area surrounding Mas Nou golf course. Since 1993, EC values in summer effluent range from 1.2 dS/m to 1.5 dS/m.
ADVERTISEMENTS:
However, major variations of effluent EC values have been observed occasionally, due to either seawater intrusion through the coastal sewerage system — increasing EC values or storm water runoff reaching the WWTP—decreasing EC values. In general, and despite irrigation with high EC water during the first years, soil salinity has not increased except for two poor- drainage areas located in fairways 12 and 15.
The corrective measures taken both in the water supply system and the sewerage system, and the evolution experienced by reclaimed water salinity clearly illustrate the important role that both potable water quality and sewerage control measures can play in achieving a reclaimed water of the best agronomical quality.
Considering that conventional wastewater secondary treatment does not modify the salinity of the influent, it is of utmost importance to plan water reclamation and reuse effluents from areas of good potable water quality and to protect the sewerage system from receiving waters of high salinity, through inadequate discharges or infiltration-inflow from surrounding brackish aquifers.
Plant Nutrients:
Table 1 shows that secondary effluent from Castell-Platja d’ Aro WWTP is a water rich in nutrients, specially during the summer, with values reaching consistently 40-45 mg NH3-N/L and 8-10 mg P04-P/L. However, Table 3 indicates that, as irrigation water is pumped from either storage pond, nutrients concentrations may show a marked variation from the values measured in the reclaimed effluent, depending basically on the hydraulic residence time (HRT) of water in the pond.
As it could be expected, nutrients concentrations in Pond 1, which receives reclaimed effluent from the WWTP, are greater than in Pond 2, which receives only water from Pond 1. Annual mass balances of nitrogen and phosphorus in both ponds show similar losses—as percentages of the nutrients mass entering each pond -, except for soluble orthophosphate in Pond 1.
As a result of the nutrients loss experienced by the reclaimed effluent during its flow through the water storage system, irrigation water from both ponds has very low nutrients concentration in early spring, whereas in summer it shows a considerable increase in nutrients because the reclaimed effluent inflow reaches both its maximum flow rate and nutrients concentration.
ADVERTISEMENTS:
This increase in nutrients concentration is specially evident in Pond 1 during the summer, where an HRT of 9 days (less than 7 days from mid-July to mid-August) results in a pond water quality very similar to that of reclaimed effluent.
The important variations experienced by nutrients concentration in both ponds, together with the different irrigation rates applied during each season, result in a very irregular turf fertilization rate; furthermore, the important differences between the water application rates imposed by the sprinkler irrigation system in the areas of influence of the two storage ponds further preclude a uniform turf fertilization rate (Table 4).
Although, on an annual basis, nutrients contributions are quite similar to nutrients requirements in the turi area irrigated with water from Pond 1, monthly mass balances show important disparities, with over-fertilization in summer and insufficient fertilization in winter. Nevertheless, if very little or no mineral fertilizer is applied after April, turf is able to overcome the fertilizer excess with no evident damage or important quality loss.
To reduce the unbalance between seasonal nutrients contributions and turf requirements, a new water and pond management system has been proposed.
ADVERTISEMENTS:
To optimize both water and nutrients application, the source of irrigation water should be the same for the all the golf course turf, and it should be possible to choose at a given time among the different water sources available at the golf course:
(a) Reclaimed effluent coming from the WWTP,
(b) Water from Pond 1, or
(c) Water from Pond 2, in the case of Mas Nou golf course.
Water source selection would be made based on nutrients concentrations in irrigation water and nutrients requirements of the turf. In winter months, when irrigation is scarce, reclaimed effluent will be more suitable for the turf because its nutrients concentrations are greater than those of pond water; in summer, when irrigation rates are the highest, the choice will be water from Pond 2 (the last in a series of interconnected ponds) because its nutrients concentrations are the lowest and nutrients contributions of irrigation water will be enough to satisfy turf requirements. Water from Pond 1 would be selected during the spring and fall seasons.
An added benefit of the proposed strategy is a lower risk of aquifer contamination with nitrates, by preventing the deep percolation of the excess amounts of nitrogen species that otherwise are applied during the summer period when both irrigation rates and nitrogen concentration in irrigation water are the highest.
The relatively low iron contributions of reclaimed water, as compared to those of nitrogen and phosphorus, have resulted in occasional spots of ferric chlorosis in the Mas Nou golf course. Those incidents have been recovered with localized application of iron compounds.
As reported by Mancino and Pepper (1994), the main effect of using reclaimed water for turf irrigation is late summer ferric chlorosis due to both an excess of nitrogen in the irrigation water, which stimulates plant growth beyond the capacity of the roots for iron uptake, and the formation of insoluble iron phosphate salts in high pH soils.
However, according to Hayes (1990), ferric chlorosis may be easily corrected with foliar applications of iron sulfate or iron chelates. Confirming Hayes’ (1990) conclusions, plot trials with iron sulfate (7 g FeSO4/m2, 18,5% Fe) and iron chelates (10 g/m2, 6% Fe) on turf areas affected by ferric chlorosis have shown an evident recovery in approximately two weeks.
Other turf nutritional deficiency observed is the lack of potassium and occasionally of nitrogen, which usually become evident when irrigation with reclaimed water is totally or substantially discontinued for a certain time due to high rainfall or to low water needs. However, correct mineral fertilizer applications help the turf to recover its adequate nutritional status.
Furthermore, some fungal infections have been observed in the fall season, mainly due to Puccinia coronata, Marasmius oreades, Laetisaria fusiformis, Sclerotinia homeocarpa and Fusarium roseum, which are favoured by excessive nitrogen contributions with irrigation water during the summer. Fungicide applications have been used to prevent and eliminate those infections.
Significant savings in fertilizers cost can be achieved by irrigation with reclaimed water. The savings achieved at Mas Nou golf course have been calculated assuming that all the nitrogen applied with irrigation water had to be provided as a mineral fertilizer.
Although this may be considered a questionable assumption, particularly in the light of the excess nitrogen applications observed during the summer season, preliminary cost estimations for 1994 indicate that an improved irrigation strategy (using reclaimed effluent during the winter) has made practically unnecessary the addition of mineral fertilizer while maintaining the golf course in excellent conditions all year round.
The result shown in Table 5 are based on the nitrogen contribution and the cost of mineral fertilizer for each year. As Table 5 indicates, the economic value of nitrogen contributions by reclaimed water have varied between US$ 9,000 and US$ 33,000 from 1991 to 1994, which represents a unit savings of 0.06-0.12 US $/m3 of reclaimed effluent.
Heavy Metals:
The heavy metals content of reclaimed water was determined during 1992 both in the reclaimed effluent and in the water of both ponds. All metals analyzed (cadmium, chromium, copper, lithium, manganese, nickel and lead) had concentrations lower than the limits recommended for irrigation waters which had to be used during a minimum of 20 years. This is in accordance with the predominantly residential character of the sewerage catchment area.
Public Health Assessment:
Systematic weekly analyses of indicator bacteria in reclaimed water entering Pond 1 show that faecal coliforms and faecal streptococci concentrations lower than 100 cfu/100 mL can be consistently achieved with chlorine doses ranging from 25 to 45 mg C\JL.
When secondary effluent turbidity remains stable and below 15 NTU, as observed during the summer of 1994, most samples show no detectable faecal coliforms or streptococci colonies in 100 mL, which represents more than 6 log removal for faecal coliforms (Table 6).
Despite these high disinfection rates, a certain regrowth of what seems to be indicator bacteria has been observed occasionally in the ponds. Although confirmatory testing for those indicator bacteria-like colonies was not within the scope of this project, it is suspected that they might be microorganisms other than indicator bacteria, but able to grow in the selective culture media used, specially in m-FC Broth Base.
Although regular tests for Salmonella enterica in secondary effluent were always positive, similar tests in water from both ponds did never detect the presence of Salmonella enterica, even with regrowth colonies concentrations up to 1 000 cfu/100 mL in faecal coliform tests.
It has to be pointed out that sunlight radiation, and daily variations of pH (up to 9, 4 in July and August) and dissolved oxygen concentration in the ponds are unfavourable conditions for the survival of microorganisms originating in the digestive tract of warm-blooded animals, such as Salmonella enterica and faecal coliforms and faecal enterococci indicators.
To improve the reliability and the disinfection performance of the water reclamation process, a 4,000 m3/day multi-media two-stage rapid sand filtration process has been designed and it is expected to become operational by the end of 1995. The more stable and lower turbidity values expected in the filtered effluent should contribute to a more efficient use of the disinfectant while achieving a better and more consistent microbiological quality of the reclaimed water.
Aesthetic Assessment:
Water extraction from the two storage ponds takes place from the bottom layers, through a pipeline connected to a nearby wet well, from where it is pumped into the irrigation system. This design arrangement has resulted in oxygen depletion and odour generation in irrigation water, particularly during the warm period from June to September.
To prevent oxygen depletion in bottom layers and its subsequent odour generation and spreading during onset of irrigation, especially in the residential area nearby Pond 1, an electric motor-driven aerator (AIRE-O2, Aeration Industries International, Inc., Minneapolis, MN, USA) has been installed; by introducing fine air bubbles directly into the deep layers of the pond (4-5 m) and also by mixing top oxygen-rich waters with bottom oxygen-depleted waters, offensive odours have been totally eliminated. Considering the reliability and efficiency of this device in controlling odours during the summer period, a second device will be installed in Pond 2 during 1995.
Systematic control of mosquito populations has been achieved with a suspension of enthomopathogenic Bacillus thuringiensis var. israeliensis (SKEETAL, Biokontrol Novo Industry A/S, Denmark). Mosquito larvae have been detected particularly during spring and summer in the shores of Pond 1 and the wet wells of both ponds. Regular cutting and removal of emerging vegetation on pond sides has proved an effective measure to prevent the spreading of mosquito populations.
Management of Reclaimed Water:
Disinfection of secondary effluent was initially (1989-1993) the responsibility of golf course operators, under the supervision of CCB technical personnel. The technical requirements of the process and the lack of reliability of disinfectant supply made quite evident to the Consorci de la Costa Brava the need to discontinue that management strategy.
A contractual agreement was established in 1993 between CCB and Mas Nou golf course, by which CCB takes the responsibility of providing reclaimed water of adequate quality and Mas Nou golf course satisfies the cost of reclaiming the secondary effluent.
Included in that cost is an assistance service by technical staff of the CCB, by which the water user is regularly informed about the chemical and microbiological quality of the reclaimed water (with prompt notice in case of high electrical conductivity values) and the estimated fertilization rates achieved by using the reclaimed water, either directly from the plant or from the two storage ponds.
Close and regular collaboration with the green keeper has resulted in a more favourable attitude to the water quality requirements of reclaimed effluent and a more effective and economical fertilization program, preventing the occasional turf over-fertilization episodes observed in previous years.
The green- keeper perception has gradually evolved from a passive acceptance of the requirements of using reclaimed water to a positive recognition of the benefits derived from using this alternative source of water, that he expresses clearly to golf course visitors, landscape professionals and managers of golf courses interested in irrigation with reclaimed water.
Based on this demonstration project, and following well established criteria, the CCB has prepared a preliminary water reclamation and reuse program for the 1995-98 period, covering specific areas of 16 municipalities of Costa Brava, with the objective of promoting agricultural irrigation, golf course irrigation, landscape irrigation and environmental restoration of rivers and wetlands. The maximum annual flow of secondary effluent expected to be reclaimed by 1998 is 15 hm3/year, which would be applied to 2,600 ha of agriculture and landscape land.
