Waste Water Treatment Plants - We have the technology
“Heredity is a strong factor, even in architecture. Necessity first mothered invention. Now invention has little ones of her own, and they look just like grandma.”
E. B. White. Most architects are fascinated by “mechanized buildings” - power stations, windmills and wells. Given these current dry times, a review of water purification facilities - some of which demonstrate simple, good architecture - may be timely.
In principle, the law charges anyone who creates sewage to purify and dispose of it appropriately, without causing sanitary or environmental hazards. We simple citizens are not equipped to purify our own sewage, so the Local Municipality Law of 1962 places the burden on local authorities. who must ensure that sewage disposal is conducted without polluting water and land resources nor serve or attracting mosquitoes, flies and other pests. To make the job palatable, local authorities are legally entitled to levy fees and taxes on its citizens and businesses, to fund construction of sewerage systems and treatment plants.
Those thirsty for facts and figures may be interested in learning that in Israel, total annual private and industrial water consumption reaches 500 million cubic meters. Of these 325 mcm undergoes purification, but only 125 mcm are processed for reuse. The rest is discharged into the Mediterranean. The agricultural sector in Israel is the greatest consumer of water (1,252 mcm, at subsidized rates). If the 200 mcm of purified water were utilized for irrigation purposes, the future of the Sea of Galilee, currently on the verge of ecological collapse due to over-pumping, could be ensured.
The Water Commissioner has at his disposal some $200 million for waste water utilization. Its use, however, is conditional on the building of sufficient reservoirs to store winter rainfall. Until then, discharge of purified water directly into the sea will continue.
Waste water treatment plants must meet two parallel challenges: the prevention of environmental pollution and the enrichment of national resources. This is not only essential but also feasible. The valuable byproducts of sewage purification can be fully utilized - organic fertilizer for enrichment of soil, clear gray water for irrigation, and methane gas as a source of energy.
How it works
The treatment plants separates the solids, less than half a percent of sewage, and purifies the liquids through a process of primary and secondary filtering, primary sludge settling, sorting, aeration, secondary sludge settling and treatment and drying of the sludge for disposal. Facilities are generally based on controlled physical-biological processes. Treatment begins with the simple separation and settling, graduating to processes based on higher cost-efficiency ratios.
Mechanical filters constructed of horizontal poles separate rags, paper, plastic and other substances, which are then compressed and transferred to containers. The sewage is transferred to sand traps where the heavier substances such as sand and seeds settle before being rinsed and collected into containers.
Activated sludge process
This calls for a primary sludge settling process in oval concrete pools; rakes at the bottom collect the organic substances. The next stage is an anaerobic process in the aeration secondary settling pools. Colonies of bacteria develop, nourished by the organic substances and oxygen integrated into the sewage through a constant stirring process. In the settling pool, the colonies settle to the bottom as active sludge, while the clear gray water lies at the surface. The active sludge is collected and most of it is returned to the aeration pool, in order to thicken the concentration of germs and make the process more efficient. The remaining sludge is transferred to the treatment pool.
This method is based on an activated sludge process combined with extended aeration. It omits the primary settling stage and facilitates full oxidation of organic substances which renders small quantities of sludge, calling for a less complicated treatment prior to disposal. While this processes calls for higher energy consumption, the omission of the primary settling stage and the reduced quantities of sludge simplifies the process and reduces the treatment plant’s running costs. The continuous oxidation pools have canals connected in a line with aerators which supply oxygen and drive the mixed liquids along the canals. Oxygen is added a second time, and in each canal different nitrogen deterring processes occur.
The sludge produced is 75% organic matter which can be used, with simple additional treatment, as a source of energy or as organic fertilizer. In treatment plants utilizing the activated sludge method, sludge is stabilized through a process of organic digestion. The primary and remaining sludge is transferred to the digester pools for transformation into methane gas, carbon dioxide and water over a period 15-20 days. To facilitate the disposal of sludge in trucks or containers, it must be dried and transformed into solid waste on condition it is free of toxic matter and heavy metals which are common components of industrial waste.
Given that appropriate technological treatment allows for retention of some 98 per cent of sewage water available for recycling to agriculture, industry, or irrigation of public parks, the country’s high quality drinking water should be exclusively for domestic consumption. We have the technology to turn the country’s waste water into purified, odorless and recyclable water for our fields, orchards, parks, and lawns. Or, we can continue to throw it all away into the Mediterranean Sea.