Conception and Design

The Dan Region Reclamation Project of municipal wastewater is the biggest project of its kind in the world. It consists of facilities for treatment and groundwater recharge of municipal wastewater from the Dan Region Metropolitan Area - an eight-city conglomerate centred around Tel Aviv, Israel, with a combined population of about 1.5 million inhabitants. The project was created for the purpose of collection, treatment and disposal of municipal wastewater. The reclaimed effluent, after a relatively long detention period in the underground aquifer, is pumped for unrestricted agricultural use, irrigating the arid Negev (the southern part of Israel). A general scheme of the project is given in figure 1. The project was established in the 1960s, and has been growing continuously. At present, the system collects and treats about 110 x 10⁶ m³ per year. Within a few years, at its final stage, the system will handle 150 to 170 x 10⁶ m³ per year.

Figure 1. Dan Region Sewage Reclamation Plant: layout

Sewage treatment plants are known to create a multitude of environmental and occupational health problems. The Dan Region project is a unique system of national importance that combines national benefit together with considerable saving of water resources, high treatment efficiency and production of inexpensive water, without creating excessive occupational hazards.

Throughout the design, installation and routine operation of the system, careful consideration has been given to water sanitation and occupational hygiene concerns. All necessary precautions have been taken to ensure that the reclaimed wastewater will be practically as safe as regular drinking water, in the event that people accidentally drink or swallow it. Similarly, appropriate attention has been given to the issue of reducing to the minimum any potential exposure to accidents or other biological, chemical or physical hazards that may affect either the workers at the wastewater treatment plant proper or other workers engaged in the disposal and agricultural use of the reclaimed water.

At Stage One of the project, the wastewater was biologically treated by a system of facultative oxidation ponds with recirculation and additional chemical treatment by a lime-magnesium process, followed by detention of the high-pH effluent in “polishing ponds”. The partially treated effluent was recharged to the regional groundwater aquifer by means of the                                                                                                                         Soreq spreading basins.

At Stage Two, the wastewater conveyed to the treatment plant undergoes mechanical-biological treatment by means of an activated-sludge process with nitrification-denitrification. The secondary effluent is recharged to the groundwater by means of the spreading basins Yavneh 1 and Yavneh 2.

The complete system consists of a number of different elements complementing each other:

• a wastewater treatment plant system, comprised of an activated-sludge plant (the biomechanical plant), which treats most of the wastes, and of a system of oxidation and polishing ponds used mostly for treatment of excess sewage flows

• a groundwater recharge system for the treated effluent, which consists of spreading basins, at two different sites (Yavneh and Soreq), that are intermittently flooded; the absorbed effluent passes through the soil’s unsaturated zone and through a portion of the aquifer, and creates a special zone that is dedicated to complementary effluent treatment and seasonal storage, which is called SAT (soil-aquifer-treatment)

• networks of observation wells (53 wells all together) which surround the recharge basins and allow the monitoring of the efficiency of the treatment process

• networks of recovery wells (a total of 74 active wells in 1993) which surround the recharge sites

• a special and separate reclaimed water conveyance main for unrestricted irrigation of agricultural areas in the Negev; this main is called “The Third Negev Line”, and it complements the water supply system to the Negev, which includes another two major fresh water supply main lines

• a setup for chlorination of the effluent, which consists, at present, of three chlorination sites (two more to be added in the future)

• six operational reservoirs along the conveyance system, which regulate the amounts of water pumped and consumed along the system

• an effluent distribution system, composed of 13 major pressure zones, along the effluent main, that supply the treated water to the consumers

• a comprehensive monitoring system which supervises and controls the complete operation of the project.

Description of the Reclamation System

The general scheme of the reclamation system is presented in figure 1 and the flow diagram in figure 2. The system consists of the following segments: wastewater treatment plant, water recharge fields, recovery wells, conveyance and distribution system, chlorination setup and a comprehensive monitoring system.

Figure 2.  Flow diagram of Dan Region Project

The wastewater treatment plant

The wastewater treatment plant of the Dan Region Metropolitan Area receives the domestic wastes of the eight cities in the region, and also handles part of their industrial wastes. The plant is located within the Rishon-Lezion sand dunes and is based mostly on secondary treatment of the wastes by the activated-sludge method. Some of the wastes, mostly during peak-flow discharges, are treated in another, older system of oxidation ponds occupying an area of 300 acres. The two systems together can handle, at present, about 110 x 10⁶ m³ per year.

The recharge fields

The treatment plant effluents are pumped into three different sites located within the regional sand dunes, where they are spread on the sand and percolate downward into the underground aquifer for temporary storage and for additional time-dependent treatment. Two of the spreading basins are used for recharge of the mechanical-biological treatment-plant effluent. These are Yavneh 1 (60 acres, located 7 km to the south of the plant) and Yavneh 2 (45 acres, 10 km south of the plant); the third basin is used for recharge of a mixture of the oxidation ponds effluent and a certain fraction from the biomechanical treatment plant that is required in order to improve the quality of the effluent to the necessary level. This is the Soreq site, which has an area of about 60 acres and is located to the east of the ponds.

The recovery wells

Around the recharge sites there are networks of observation wells through which the recharged water is re-pumped. Not all of the 74 wells in operation in 1993 were active during the whole project. In 1993 a total of about 95 million cubic metres of water were recovered from the system’s wells and pumped into the Third Negev Line.

The conveyance and distribution systems

The water pumped from the various recovery wells is collected into the conveyance and distribution system of the Third Line. The conveyance system is composed of three sections, having a combined length of 87 km and a diameter ranging from 48 to 70 inches. Along the conveyance system six different operational reservoirs, “floating” on the main line, were constructed, in order to regulate the water flow of the system. The operational volume of these reservoirs ranges from 10,000 m³ to 100,000 m³.

The water flowing in the Third Line system was supplied to the customers in 1993 through a system of 13 major pressure zones. Numerous water consumers, mostly farms, are connected to these pressure zones.

The chlorination system

The purpose of the chlorination that is carried out in the Third Line is “breakage of the human connection”, which means elimination of any possibility for existence of micro-organisms of human origin in Third Line water. Throughout the course of monitoring it was found that there is a considerable increase of fecal micro-organisms during the stay of the reclaimed water in the water reservoirs. Therefore it was decided to add more chlorination points along the line, and by 1993 three separate chlorination points were routinely operating. Two more chlorination points are to be added to the system in the near future. The residual chlorine ranges between 0.4 and 1.0 mg/l of free chlorine. This method, whereby low concentrations of free chlorine are maintained at various points along the system rather than a single massive dose at the beginning of the line, secures the breakage of the human connection, and at the same time enables fish to live in the reservoirs. In addition, this chlorination method will disinfect the water in the downstream sections of the conveyance and distribution system, in the event that pollutants entered the system at a point downstream from the initial chlorination point.

The monitoring system

Operation of the reclamation system of the Third Negev Line is dependent upon routine functioning of a monitoring setup which is supervised and controlled by a professional and independent scientific entity. This body is the Research and Development Institute of the Technion - Israel Institute of Technology, in Haifa, Israel.

The establishment of an independent monitoring system has been a mandatory requirement of the Israeli Ministry of Health, the local legal authority according to the Israeli Public Health Ordinance. The need for establishing this monitoring setup stems from the facts that:

1. This wastewater reclamation project is the biggest one in the world.
2. It comprises some non-routine elements that have not as yet been experimented with.
3. The reclaimed water is to be used for unlimited irrigation of agricultural crops.

The major role of the monitoring system is therefore to secure the chemical and sanitary quality of the water supplied by the system and to issue warnings regarding any change in the water quality. In addition, the monitoring setup is conducting a follow-up of the complete Dan Region reclamation project, also investigating certain aspects, such as the routine operation of the plant and the chemico-biological quality of its water. This is necessary in order to determine the adaptability of the Third Line water for unlimited irrigation, not only from the sanitary aspect but also from the agricultural viewpoint.

The preliminary monitoring layout was designed and prepared by the Mekoroth Water Co., the major Israeli water supplier and the operator of the Dan Region project. A specially appointed steering committee has been reviewing the monitoring programme on a periodic basis, and has been modifying it according to the accumulated experience gained through the routine operation. The monitoring programme dealt with the various sampling points along the Third Line system, the various investigated parameters and the sampling frequency. The preliminary programme referred to various segments of the system, namely the recovery wells, conveyance line, reservoirs, a limited number of consumer connections, as well as the presence of potable water wells in the vicinity of the plant. The list of parameters included within the monitoring schedule of the Third Line is given in table 1.

Table 1. List of investigated parameters

Recovery wells monitoring

The sampling programme of the recovery wells is based upon a bi-monthly or tri-monthly measurement of a few “indicator-parameters” (table 2). When the chlorides concentration at the sampled well exceeds by more than 15% the initial chlorides level of the well, it is interpreted as a “significant” increase of the share of the recovered effluent within the underground aquifer water, and the well is transferred into the next category of sampling. Here, 23 “characteristic-parameters” are determined, once every three months. In some of the wells, once a year, a complete water investigation, including 54 various parameters, is carried out.

Table 2. The various parameters investigated at the recovery wells

Conveyance system monitoring

The conveyance system, the length of which is 87 km, is monitored at seven central points along the wastewater line. At these points 16 different parameters are sampled once per month. These are: PHFD, DO, T, EC, SS₁₀, SS₅₅, UV, TURB, NO₃ ⁺, PTOT, ALKM, DOC, TOTB, TCOL, FCOL and ENTR. Parameters which are not expected to change along the system are measured at two sampling points only - at the beginning and at the end of the conveyance line. These are: Cl, K, Na, Ca, Mg, HARD, B, DS, SO₄ ^–2, NH₄ ⁺, NO₂ ^– and MBAS. At those two sampling points, once a year, various heavy metals are sampled (Zn, Sr, Sn, Se, Pb, Ni, Mo, Mn, Li, Hg, Fe, Cu, Cr, Co, Cd, Ba, As, Al, Ag).

Reservoirs monitoring

The monitoring setup of the Third Line reservoirs is based mostly on examination of a limited number of parameters which serve as indicators of biological development in the reservoirs, and for pinpointing the entry of external pollutants. Five reservoirs are sampled, once per month, for: PHFD, T, DO, Total SS, Volatile SS, DOC, CLRL, RSCL, TCOL, FCOL, STRP and ALG. At these five reservoirs Si is also sampled, once per two months. All these parameters are also sampled at another reservoir, Zohar B, at a frequency of six times per year.

Summary

The Dan Region Reclamation Project supplies high-quality reclaimed water for unrestricted irrigation of the Israeli Negev.

Stage One of this project is in partial operation since 1970 and in full operation since 1977. From 1970 to 1993, a total raw sewage amount of 373 million cubic metres (MCM) was conveyed to the facultative oxidation ponds, and a total water amount of 243 MCM was pumped from the aquifer in the period 1974–1993 and supplied to the South of the country. Part of the water was lost, mostly due to evaporation and seepage from the ponds. In 1993 these losses amounted to about 6.9% of the raw sewage conveyed to the Stage One plant (Kanarek 1994).

The mechanical-biological treatment plant, Stage Two of the project, has been in operation since 1987. During the 1987-1993 period of operation a total raw sewage amount of 478 MCM was conveyed to the mechanical-biological treatment plant. In 1993 about 103 MCM of water (95 MCM reclaimed water plus 8 MCM potable water) were conveyed through the system, and used for unlimited irrigation of the Negev.

The recovery-wells water represents the underground aquifer water quality. The aquifer water quality is changing all the time as a result of the percolation of effluent into it. The aquifer water quality approaches that of the effluent for those parameters that are not influenced by the Soil-Aquifer Treatment (SAT) processes, while parameters that are affected by the passage through the soil layers (e.g., turbidity, suspended solids, ammonia, dissolved organic carbon and so on) show considerably lower values. Noteworthy is the chloride content of the aquifer water, which increased within a recent four-year period by 15 to 26%, as evidenced by the changing water quality in the recovery wells. This change indicates the continuous replacement of aquifer water by effluent having a considerably higher chloride content.

The quality of the water in the six reservoirs of the Third Line system is influenced by biological and chemical changes that occur within the open reservoirs. The oxygen content is increased, as a result of photosynthesis of algae and due to dissolution of atmospheric oxygen. Concentrations of various types of bacteria are also increased as a result of random pollution by various water fauna residing near the reservoirs.

The quality of the water supplied to the customers along the system is dependent upon the quality of water from the recovery wells and the reservoirs. Mandatory chlorination of the system’s water constitutes an additional safeguard against erroneous use of the water as potable water. Comparison of the Third Line water data with the requirements of the Israeli Ministry of Health regarding quality of wastewater to be used for unlimited agricultural use shows that most of the time the water quality fully satisfies the requirements.

In conclusion it might be said that the Third Line wastewater recovery and utilization system has been a successful environmental and national Israeli project. It has solved the problem of sanitary disposal of the Dan Region sewage and at the same time it has increased the national water balance by a factor of about 5%. In an arid country such as Israel, where water supply, especially for agricultural use, is quite limited, this is a real contribution.

The costs of the recharge operation and maintenance of the reclaimed water, in 1993, was about 3 US cents per m³ (0.093 NIS/m³).

The system has been operating since the late 1960s under strict surveillance of the Israeli Ministry of Health and of Mekoroth’s occupational safety and hygiene department. There have been no reports of any occupational disease resulting from the operation of this intricate and comprehensive system.

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