New Water/Wastewater Technology Put to Work in French Plants
Written By Ian Lisk, Editorial Director
As a member of a small delegation of trade journalists (four from the United States and one from England) invited by Suez Lyonnaise des Eaux to visit France in October, I was privileged to attend the inauguration of an impressive water treatment plant designed, built and operated by that company. This event at the beautiful (the only word to describe it) Vignieux sur Seine plant, which was attended by some 300 guests, including France's Minister of the Environment Dominique Voynet, was covered in a Water Online article which appeared on our site October 20.
The article described how this very old plant has been upgraded in stages and is now a 14.5-million-gallon-per-day (mgd) facility whose modern process train was designed by Lyonnaise to use some of the firm's latest technologies. These include inclined plate settlers, deep tube ozonation, the addition of powdered activated carbon, and as a final polishing step, a membrane-based ultrafiltration process known as Cristal. This facility is said to be the largest membrane plant of its kind built anywhere in the world to date.
Preceded by the combination of conventional and advanced processes, the ultrafiltration section receives water which already has been purified to a high degree. Consequently the 224 Cristal modules, each containing about 15,000 hollow, porous fibers in seven 30-cm-diameter, 1.3-meter-long bundles, can go to work to remove any remaining micro-pollutants, including bacteria, viruses and organic pesticides. They do not, however, remove the desirable natural mineral salts in the water. This process is fully automated and the modules, which are manufactured by the subsidiary Aquasource, are each cleaned by backwashing on a programmed schedule every 45 minutes. Every three months the modules are given a very thorough cleaning, which is said to ensure a five-year life for the membranes.
The Cristal process is the result of many years of research by members of the Lyonnaise group, particularly CIRSEE, the International Research Center of Lyonnaise des Eaux, which has been responsible for a number of breakthrough technology developments for treating water or wastewater. To investigate the latter, about 18 months ago CIRSEE opened a research-oriented facility at the Evry wastewater treatment plant near Paris on the River Seine. This investment of about $1.6 million makes it possible to carry out pilot scale process development and improvement work on the site of an operating wastewater treatment plant under real influent conditions if required. I'll describe what's going on at Evry later.
Colombes: A Benchmark Wastewater Treatment Plant
What got my attention most during our French trip—even though Vignieux sur Seine and Evry were impressive—was the huge construction project at Colombes where a new style of wastewater treatment plant is being built by Lyonnaise subsidiary Degrémont. As a former engineer in industry who took part in the construction and start-up of three manufacturing plants, I was impressed by the scope and scale of this project.
Being built in this town north of the French capital for the City of Paris Sanitation Authorities (SIAAP), the plant is on the site of a former, much smaller treatment facility. When the bid was awarded in October 1992 the cost was estimated at 2.2 billion francs (approximately $355 million at today's exchange rate). Approval to proceed was given in November of 1993, initial fill and the beginning of startup is scheduled for April of 1998, with acceptance planned for next November.
What is so remarkable about this very large plant is its innovative concept—it is a multi-level, entirely enclosed facility. Much of it is underground with its 4-story superstructure covering just 75 percent of the underground area. All functions and activities will proceed in the indoor space, including a very large advanced flue gas treatment facility for exhaust gases and odor control that is installed in a bay almost 100 ft high. Also, the noise of pumps and motors and other equipment will be contained within the building. The design takes into account these environmental nuisance factors, places the plant on a footprint that is small for such a large process capacity, and architecturally has made the building as unobtrusive as possible. When finished the facility will have park-like surroundings with landscaping and public footpaths.
Designed to meet the European effluent standards that apply to "sensitive areas," the plant is situated on the bank of the Seine and close to residential and commercial communities which have combined sewer collection systems. These standards include the virtual elimination of organic matter (BOD5 < 25mg/l and COD < 90 mg/l) and nitrogen, by nitrification up to 12C then denitrification (TN < 10 mg/l). But perhaps the most important performance feature of the plant will be its ability to adapt to a wide variety of climatic conditions, with little degradation of effluent quality as influent flow rates rise because of rainfall . Dry weather flow can be as low as approximately 65 mgd, while storm events in the area can increase the flow to as much as 275 mgd.
The process train incorporates advanced versions of generally traditional wastewater systems. After pre-treatment nine of Degrémont's Densadag units handle the task of removing suspended solids and phosphorus and also reduce BOD substantially. Densadag is a compact physical/chemical sedimentation system which incorporates inclined plates for rapid settling.
Following this section are three bio-filter stages—24 Biofor units, 29 Biostyr units, and 12 more Biofor units. The Degrémont-developed Biofor is a high-rate, upflow biological filter with bottom air injection and proprietary filter media which also is a space-saving compact design.
The Colombes plant is the first to use these systems in several different ways. With dry weather influent flows, the units operate in series to eliminate residual BOD values, nitrify, then denitrify with the addition of methanol. When moderate rainfall occurs, the first and third biofilter stages run in parallel to eliminate organics, while the second stage carries out nitrification in series with the first. In high flow conditions caused by events, all three biolfilter banks operate in parallel to reduce BOD and nitrify simultaneously.
Sludge from the biofilter section is thickened by a flotation process. Then the thickened product is run through centrifuges and subsequently lime-treated for agricultural use or incinerated in two 15-ft diameter furnaces. Conditioned sludge is taken away by barges on the nearby Seine, and the off-gases from the furnaces are dealt with in the air pollution control system.
The Evry Research Facility
Our small group of visitors was taken to Evry on the second afternoon of the trip to hear about the latest work there and view the pilot plants. The processes being investigated are the traditional "free culture" biological systems, fixed film or "biofilm" cultures, and Lyonnaise's Membrane BioReactor (MBR), which the company believes is one of its most innovative technological developments.
The MBR system combines an activated sludge process with the physical separation capabilities of ultrafiltration (UF) membranes. The UF process takes the place of traditional clarification by sedimentation, which is seen as the weak link in classical processes. As described above in the opening paragraphs about the Vignieux sur Seine water treatment plant, these UF membranes provide a total barrier to bacteria, viruses, parasites, and residual suspended solids. The MBR effluent is said to be 100 percent disinfected, and the system has been shown to be competitive for treating and recycling industrial wastewater in five pilot scale investigations.
Currently the objective at the Evry facility is to make the MBR process cost effective for installation in large wastewater facilities. An important feature of the system is its compact footprint. With a smaller activated sludge section, the elimination of the settling step, and the compactness of the membrane modules, a plant designed for this process could be one third the size of a conventional facility, according to CIRSEE.
Another CIRSEE development under pilot-scale study is a fixed film biological process named TURBO for the reduction of carbonaceous and nitrate concentrations in urban wastewater. This is a three-stage system which uses granular media of large surface area and a lower-than-water density as the biofilm base. The granules are circulated in the wastewater stream and exposed to air injection in a two-compartment "gas-lift" reactor. An important advantage of the proprietary media is that they are manufactured, hence their quality and characteristics can be controlled to a specification. TURBO is said to provide a higher level of biological activity compared to the classical activated sludge system, and with the same nitrifying capacity takes up only about 25 percent of the space needed by the older process. It also is said to be extremely stable and reliable in operation.
Some of the studies underway or planned at Evry will investigate two-phase anaerobic digestion, oxygen transfer, trickling filters, activated sludge optimization and enhancement, and primary sludge hydrolysis. This pilot facility is just one illustration of how Suez Lyonnaise des Eaux approaches the task of finding new ways to treat water and wastewater. Yet the Evry budget is only a small portion of what the company spends on its entire range of research and development activities.
The company presently commits nearly $50 milliom annually for R&D work on water and the environment, with 45 percent of that figure set aside for wastewater treatment. This is said to be the largest budget devoted to research and development by a privately-owned water services company in the world. And one of the company's present objectives is to develop treatment process trains that combine conventional and new technology cost effectively. Some of the successes so far are well demonstrated in the facilities that our journalist group was able to visit.