WASTEWATER TREATMENT

Municipal and industrial sewage remain the major culprits responsible for Israel’s polluted rivers, but increased public awareness and legal proceedings, are making a difference. Jerusalem which discharges over 20 MCM of its sewage into the Soreq River, has agreed to accelerate its timetable for construction of a $100 million sewage treatment plant, due to pressure from the Ministry of the Environment. In Eilat, a legal suit has led to a court order obliging the city to construct a 30-kilometer pipeline to divert its sewage northward where it will be used for agricultural irrigation following treatment in a sewage plant.

While in some cases restoration efforts call for the removal of effluents from streams altogether (e.g. the Taninim river), in most cases efforts are focused on improving effluent quality to a degree which will allow recreational uses such as beaching or even fishing. Advanced treatment plants, expected to be operational within one to three years, are now under construction in Netanya, Ramat Hasharon, Hod Hasharon, Kfar Sava and Ra’anana. Their operation will significantly upgrade water quality in the Yarkon, Poleg and Alexander Rivers.

Cesspools and septic tanks, mostly in Israel’s rural sector, once constituted major sources of groundwater pollutionbut no more. Regulations promulgated in 1992, under the Water Law, prohibit the construction of new cesspools and place severe restrictions on existing ones, including timetables for their elimination. The regulations call for disconnecting sewage outlet pipes from industrial and domestic cesspools and for connecting them to a suitable sewage network.

A national program for sewage was first drawn up in 1970 and came into effect in 1973. Over $160 million were invested in the program, which comprised a large scheme for the densely-populated Tel Aviv metropolitan area, two regional projects and forty-four smaller-scale municipal projects.

Of the total volume of wastewater produced in Israel, about 90% is collected by means of central sewage systems, 80% is treated and nearly 70% is reclaimed for reuse. Of the total wastewater volume, a third is treated to a high level in the Dan Region Wastewater Treatment Project, another third is treated to lower levels and the remaining third is left more or less untreated.

The Dan Region Wastewater Project is a sophisticated system serving an area of 220 km2 which encompasses the large metropolis of Tel Aviv-Jaffa and seven other municipalities, with a population of 1.3 million. Ninety percent of the wastewater processed comes from households and 10% from industry. In 1993, 87 MCM of wastewater were treated at the plant, of which 75 MCM were recharged and about 100 MCM (together with groundwater) were supplied for agricultural irrigation in the Negev. The Dan Project provides for biological treatment of wastewater including nutrient removal

(to a level of 20-25 mg/liter BOD and 30-40 mg/liter suspended solids). The secondary effluent is then recharged into the groundwater aquifer by means of spreading sand basins for additional polishing and long-term storage. The water is eventually pumped and used for unrestricted irrigation in Israel’s arid Negev desert. The high quality of the treated water after recharge conforms to drinking water standards, but is not used for this purpose. An extensive hydrological and water quality monitoring program, carried out by means of a network of observation wells and recovery wells surrounding the recharge basins, has confirmed the high quality of the reclaimed water.

In the Haifa region, another large-scale project treats about 38 MCM of wastewater yearly. The technique uses an integrated technology of activated sludge and trickling filter which produces medium-quality effluent (60-80 BOD/SS). The effluent is piped 30 kilometers eastward to irrigation reservoirs that serve the Jezreel Valley. There water quality further improves after a long retention time in effluent reservoirs.

Throughout the country, numerous smaller municipal and local wastewater treatment systems exist. The quality of effluents produced by these treatment plants is not uniform, and all too often local authorities do not operate and maintain their facilities properly. Leakages occur from some sewerage systems as a result of blockages or the poor condition of the piping, leading to environmental nuisances. In Jerusalem, for example, some 22 MCM of effluents are discharged untreated into the Soreq River. Effluent quality of Jerusalem’s sewage is in the range of 450-500 BOD/SS.

Increased awareness is resulting in improvements in existing plants and in the planning of new treatment plants which conform to new Israeli standards on effluents. Effluent regulations, promulgated in 1993, require secondary treatment to a level of 20 mg/liter BOD and 30 mg/liter suspended solids as a minimum baseline level. Higher degrees of treatment are required by the Ministry of the Environment if effluents are to be discharged into rivers rather than for agricultural use. In such cases, nutrient removal and disinfection are prescribed.

Effluent Reuse

The combination of severe water shortage, contamination of water resources, densely-populated urban areas and highly intensive irrigated agriculture, makes it essential that Israel put wastewater treatment and reuse high on its list of national priorities. Effluents constitute the most readily available and cheapest source of additional water, and provide a viable partial solution to Israel’s water scarcity problem.

By the beginning of the next century, a significant increase in water demand is expected. As a result, the volumes of fresh water now diverted for agriculture will decrease to about half that supplied in the past. This will create an increasing demand for effluents for irrigation in the agricultural sector. By the year 2000, some 400 MCM of effluent may be reused for irrigation in agriculture; at the same time, fresh water demand for urban consumption (domestic and industrial) will increase considerably, to 730 MCM.

At the request of the Water Commission, Israel’s water planning company, Tahal, has recently prepared a national masterplan for effluent reclamation, describing present conditions and prescribing a program for future development. The plan constitutes a framework for preparation of national and regional masterplans for effluent reclamation including flow forecasts, principal treatment sites, principal schemes planned, projected interregional transfers and estimates of investments in effluent reclamation in Israel. The aim is to achieve maximum treatment in order to prevent environmental nuisances and to enable effluent reuse in agriculture throughout the country.

Studies on the human health consequences of permissible effluent irrigation have shown that no negative effects have resulted from the reuse of treated wastewater practiced in Israel over the last thirty years. The Ministry of Health maintains a permit system designed to ensure that irrigation with effluents is limited to crops such as cotton, corn for fodder, etc. Only highly treated effluents, after chlorination, are used for irrigation of orchards and other edible crops, but effluents are never used for irrigation of vegetables or other crops which may be consumed directly without cooking.

Effluent Reservoirs

The large-scale reclamation of effluents practiced in Israel makes it necessary to store the effluent in seasonal reservoirs (100 thousand to 3 million cubic meters in volume). The reservoirs, some 160 in number, are a part of numerous small reuse schemes in Israel, as well as of large-scale projects such as those in the Haifa region (the Kishon scheme). The reservoir is filled throughout the year with effluent at a relatively constant flow, while water is withdrawn only during the dry months. The reservoirs are often used as a "polishing" step in the treatment of the wastewater and are then used for the irrigation of cotton, silage and other non-food crops. During the retention period in the reservoir, which varies from two to several months, a series of physical, chemical and biological processes take place, affecting water quality.

A survey of Israel’s wastewater collection reservoirs, undertaken by the Ministry of the Environment and the Nature Reserves Authority, has revealed that while some improvement in water quality, due to self-purification processes, occurs in the reservoirs in the spring, water deterioration occurs in the autumn. The quality of the final effluents, at the point of discharge, was found to be highly variable and dependent on many factors, including:

* Retention time of the water in the reservoir: Since irrigation is undertaken in summer only, effluent quality can be very high in spring (5-30 mg/liter BOD) when the reservoir is full and retention time is long. In the later summer, however, water quality usually deteriorates and can reach values of BOD as high as 200 mg/liter;
* Level of treatment of the effluents introduced into the reservoir;
* Ratio of effluent to rainwater and spring water stored in the reservoir;
* Addition of high strength agricultural effluents to domestic effluents.

On the basis of the findings, the survey concluded that effluent reservoirs should only receive relatively high-quality effluents (20/30 mg/liter BOD/SS). Furthermore, reservoirs should not be considered integral parts of the treatment system unless long retention time is ensured by the use of a series of reservoirs before discharge of the water for irrigation purposes.

Industrial Wastewater

By law, no plant which produces industrial wastewater can be approved until it ensures adequate treatment of its wastewater prior to discharge into the municipal system. The Model By-Law for Local Authorities on the discharge of industrial wastes into the sewage system prohibits the discharge of industrial wastes into the sewage system in a manner, quantity or quality that might cause damage to the sewage system, to the flow of sewage or to the treatment process. Thus, many factories have established in-house facilities for the pretreatment of industrial sewage before their discharge into the municipal sewage system.

Current criteria for wastewater treatment levels do not take account of the level of total salts and their composition. Yet, in view of the ever-growing quantities of effluents used for irrigation purposes, high salinity levels threaten to damage agricultural soils, reduce crop yields, and in certain cases, cause groundwater salinity.

The major sources that contribute to the high sodium concentration in Israel’s wastewater include domestic and industrial water softening and the meat "koshering" process. Fresh water in Israel has a high average hardness, and water softening is performed routinely in factories for steam production, cooling towers, laundries, textile dye works and other industries. Along its passage through a city, water is enriched with salts, typically in the range of 170 mg/literalthough higher values have been reported. As a result of the release of sodium salts (mainly NaCl), sewage effluents have a higher salt content and a higher sodium adsorption ration (SAR) than the urban water supply. The average SAR increase is about two units, from 2.5 in good quality water to 5-8 in sewage water. Such an increase in the SAR is coupled by a similar increase in the exchangeable sodium percentage (ESP) of the soil.

While the model by-law on industrial wastewater discharge into the sewage system forbids the discharge of chlorides in concentrations greater than 200 milligrams per liter above their concentration in the water supplied to the plant, the provision remains unenforced. In order to circumvent discharge into the municipal sewage system, some industries discharge their salts to evaporation pools or to the sea. Plans are currently underway to enable the discharge of salts (produced as a result of water softening via the regeneration of ion exchange) from industrial plants in the Dan metropolitan area through an outfall to the sea. It is estimated that this solution will reduce chloride concentrations in effluents produced in the Dan Wastewater Treatment Plant by 50 mg/l.

The Ministry of the Environment is currently investing efforts in guiding industries to reduce salt emissions through more efficient use of the raw materials responsible for salination and through recycling for reuse. Since the regeneration of ion exchange contributes some 30% of the total quantity of sodium emitted by industry into wastewater, the ministry has completed a draft regulation, within the framework of the Water Law, to reduce salt emissions from this process. The ministry is also encouraging major contributors, such as the food and textile industries, to adopt technologies for the recycling of sodium hydroxide for reuse purposes. Several plants have begun using these technologies.

In addition to these savings and recycling measures, the ministry is also examining the following options:

* Discharge of brines into the sea;
* Replacement of sodium salts with potassium salts in ion exchange and in other industrial processes;
* Reduction of the sodium discharges resulting from the use of detergents, shampoos, dishwashing and washing machine powders (which contribute some 50% of total sodium emissions to wastewater);
* Precipitation of Ca and Mg in the effluents from ion-exchangers and recycling of the NaCl solution;
* Construction of centralized systems for the supply of soft water in industrial areas;
* The substitution of domestic ion-exchangers of the self-regenerating type by the portable type;
* Reduction of the discharge of salts by the meat "koshering" process; * New membrane technology for salt recovery.