Proceedings of the International Symposium on Efficient Water Use in Urban Areas

Case History 1: Wastewater Treatment of Greater Agadir, Morocco (Bennani, et al., 1992)

Today with its population of over 350,000, the rapidly growing Greater Agadir faces need for wastewater treatment and increased demand for water supply. The two main discharges of raw sewage, one into the port area, the other into the bed of the Souss wadi, at a few kilometers of its mouth, become less and less compatible with valuable tourist attraction.

In the Moroccan-French cooperation project, pilot wastewater treatment through dune sand infiltration-percolation is underway at Ben Sergao (a suburb of Agadir). After treatment by anaerobic stabilization pond (chemical oxygen demand, COD, of raw sewage=1,190 mg/L), the pilot wastewater treatment by infiltration-percolation plant treats 1,000 m³/d of highly concentrated effluents in five infiltration basins of 1,500 m² each, consisting of two-meter thick eolian sand. The anaerobic stabilization pond (1,500 m³ for a theoretical residence time of 2 days; depth of the pond: 3-4 m) is used to reduce suspended solids (40-50 %) and organic matter (50-60 %); thus, reducing the surface areas necessary for the infiltration basin. The basin is submerged for 8 hours and stays dry for 16 hours.

The wastewater was infiltrated at the rate of one meter per day. With this process nearly 100 percent of suspended solids and 95 percent of COD are removed, and 85 % of nitrogen is in oxidized forms and 56 % removed. Microbiological quality of raw sewage, pond effluent, and percolated water are shown in Table 1. The percolated water will be used in growing tomatoes (a vegetable extensively cultivated in the Agadir region), public gardens, and future golf courses.

Table 1. Microbiological quality of raw sewage, pond effluent, and percolated water (Bennani, et al., 1992)

		Raw sewage	Pond effluent	Percolated water	Overall removal efficiency

Fecal coliforms,		6x10⁶	5x10⁵	327	4.26 logs
	Numbers/100 mL
Fecal streptococci,		2x10⁷	1.6x10⁶	346	4.78 logs
	Numbers/100 mL
Nematode eggs,		139	32	0	100 %
	Numbers/L
Cestode egg, numbers/L		7	18	0	100 %
Total helminths egg,		214	47	0	100 %
	Numbers/L

Inasmuch as recharged groundwater may be an eventual source of potable water supply, groundwater recharge with reclaimed municipal wastewater may often involve treatment beyond the conventional secondary wastewater treatment. In the past, prior to the recent concerns about protozoan cysts, enteric viruses, and trace organics in drinking water, several apparently successful groundwater recharge projects were developed and operated using primary and secondary effluents in spreading basins. However, because of the increasing concerns for these contaminants, groundwater recharge with reclaimed wastewater normally entails further treatment following conventional secondary treatment. For example, for surface spreading operations practiced in the United States, common wastewater reclamation processes include primary and secondary wastewater treatment, and tertiary granular-medium filtration followed by chlorine disinfection. The following Case History 2 deals with infiltration percolation as a tertiary treatment to meet the World Health Organizations (WHO) microbiological standards applying to unrestricted agricultural reuse.

Case History 2: Disinfection of Secondary Effluents by Infiltration and Percolation (Brissaud, et al., 1998).

Effluent from conventional wastewater treatment cannot be directly used without disinfection for irrigation of public parks, sports fields, golf courses and edible crops. They have to be disinfected prior to reuse in order to comply with relevant regulations. When the object is to meet WHO’s unrestricted irrigation criteria (WHO, 1989), the additional treatment can be achieved through infiltration peroration. Infiltration peroration plants were intermittently fed with secondary effluents which percolate through 1.5 to 2 m unsaturated coarse sand, and recovered by underdrains.

Infiltration percolation allows oxidizing and disinfecting wastewater. These are the reasons why soil aquifer treatment is used, in Spain and France, as a tertiary treatment with the aim of removing pathogens from the effluents of conventional wastewater treatment plants. It is a low-technology method that can be used to prepare wastewater for unrestricted irrigation (Brissaud, et al., 1998). However, the reported disinfection performances provided by infiltration percolation are uneven dependent on the hydraulic loading of the system. The average water quality of secondary effluents and the percolated water is shown in Table 2.

As discussed in the previous sections, groundwater recharge with reclaimed municipal wastewater presents a wide spectrum of health concerns. It is essential that water extracted from a groundwater basin for domestic use be of acceptable physical, chemical, microbiological, and radiological quality. Main concerns governing the acceptability of groundwater recharge projects are that adverse health effects could result from the introduction of pathogens or trace amounts of toxic chemicals into groundwater that is eventually consumed by the public. Because of the increasing concern for long-term health effects every effort should be made to reduce the number of chemical species and concentration of specific organic constituents in the applied water (National Research Council, 1982 and 1994; State of California, 1987).

A source control program to limit potentially harmful constituents entering the sewer system must be an integral part of any groundwater recharge project. Extreme caution is warranted because of the difficulty in restoring a groundwater basin once it is contaminated. Additional cost would be incurred if groundwater quality changes resulting from recharge necessitated the treatment of extracted groundwater and/or the development of additional water sources.

In the United States, federal requirements for groundwater recharge with reclaimed municipal wastewater have not been established. As a consequence, water reclamation and reuse requirements for groundwater recharge are established by the state agencies such as the California Regional Water Quality Control Boards with a case-by-case determination of the project (State of California, 1978). Considerably higher wastewater treatment prior to groundwater recharge is advocated in general because of the health concerns related to potential chronic effects of trace organics, and waterborne pathogens, particularly, enteric viruses upon human health.

The proposed criteria for groundwater recharge with reclaimed municipal wastewater rightly reflect cautious attitude toward such short-term as well as long-term health concerns. Proposed Criteria (State of California, 1992) are shown in Table 3. The criteria rely on a combination of controls intended to maintain a microbiologically and chemically safe groundwater recharge operation. No single method of control would be effective in controlling the transmission and transport of contaminants of concern into and through the environment. Therefore, source control, wastewater treatment processes, treatment standards, recharge methods, recharge area, extraction well proximity, and monitoring wells are all specified.

The requirements in Table 3 are specified by "project category" which identify a set of conditions that constitute an acceptable project. An equivalent level of perceived risk is inherent in each project category when all conditions are met and enforced. Main concerns governing the acceptability of groundwater recharge projects with reclaimed municipal wastewater are that adverse health effects could result from the introduction of pathogens or trace amounts of toxic chemicals into groundwater that is eventually consumed by the public.

Level of wastewater treatment:
Primary/secondary	X	X	X	X
Filtration	X	X		X
Organics removal	X			X
Disinfection	X	X	X	X

Microbiological Considerations. Of the known waterborne pathogens, enteric viruses have been considered most critical in wastewater reuse in California because of the possibility of contracting disease with relatively low doses and difficulty of routine examination of reclaimed wastewater for their presence. Thus, essentially virus-free effluent via the full treatment process (primary/secondary, coagulation/flocculation, clarification, filtration, and disinfection) is deemed necessary by the California Department of Health Services (State of California, 1978) for reclaimed wastewater applications with higher potential exposures, e.g., spray irrigation of food crops eaten uncooked, or most of groundwater recharge applications such as Project Categories I, II, and IV in Table 3.

The wastewater treatment requirements in Table 3 are designed to provide assurance that reclaimed water is essentially pathogen-free prior to extraction from the groundwater. The pathogen, e.g., enteric viruses, removal capabilities of an individual or a combination of treatment processes have been estimated (Hultquist, et al., 1991) and the virus removals achieved by various combinations of wastewater treatment is reported in Table 4.

In addition to the treatment processes shown in Table 4, passage through an unsaturated zone of significant depth (> 3 m) reduces organic constituents and pathogens in treated effluents. At low infiltration rates of less than 5 m/day in sands and sandy loams, the rates of virus removal are approximated by a semi-log plot (k = -0.007 log/cm) against infiltration rates, resulting approximately 99.2 % or 2.1 logs removal for 3 m depth soils. The overall estimate for the removal of enteric viruses by the treatment processes, unsaturated zone, and horizontal separation (retention time in groundwater) as specified in the proposed requirements is shown in Table 5.