|Lake and Reservoir Management, 9(l):98-102. (1994) Extended Abstract of a paper presented at the 13th International Symposium of the North American Lake Management Society, Seattle, WA, Nov. 29 Dec. 4, 1993.|
Aeration And Pollutant Abatement in the Helpe Mineure River, Fourmies, France.
Robert L. Laing and Carlton J. Rausch
Fourmies, France is a rural town of 20,000 people. It has a large textile industry with spinning mills and cloth mills, a nut and bolt factory, glass bottling company, computer parts factory, a seat cover manufacturer, and several smaller industries such as restaurants, stores, bars and hotels. As the Helpe Mineure River (Fig. 1) passes through Fourmies, COD and ammonia increase, and oxygen decreases. Because of the water quality degradation, the water was in the unusable category for 43 km downstream.
In 1974, local authorities began work to restore the river. They formed the SytidicatD’Assaissement de Fourmies-Wignehies (Intermunicipal Sanitation Agency of Fourmies-Wignehies). A series of actions were initiated which included: 1. Suppress direct discharges into the river, 2. Continue to upgrade storm runoff, 3. Extend the sanitation (sewage) system, and 4. Improve the quality of treatment at the waste water treatment plant. This paper does not describe these four actions, as they comprise the traditional approach to water quality management.
Figure l. Map of the Helpe River from Fourmies, France to Etroeungt, 43 km downstream, showing test locations.
In 1990 the Syndicat conceded that oxygen was still too low after the river passed through Fourmies. This resulted from a multitude of point and nonpoint discharges to the river as it passed through the village. COD and ammonia at Fourmies were high, causing the low oxygen level. The river remained polluted for about43kindownstream,andthefaunawasclassified as 7 on the pollution index, corresponding to critical biological water quality.
The goal of this project was to reoxygenate the river from Fourmies to Rocquigny- This would reduce both COD and ammonia as the water flowed downstream. Since the river is approximately 50 cm deep, it is too shallow to aerate directly. So CLEAN-FLO proposed to dam the river, divert it into a retention pond, aerate the water, and return it to the river. Reoxygenation of the river was then instituted using the CLEAN-FLO process of inversion/oxygenation. This paper will discuss the reoxygenation of the river and its results
As the river flows from Foumies to Wignehies, it takes an average of 5.5 hours. Within this time, natural aeration of the river increases DO to 50 percent of saturation. The retention pond was designed to have a minimum retention time of 5.5 hours at maximum flow. This would increase DO to 50% saturation at the discharge point. Ammonia and COD would not be significantly reduced in the retention lagoon. However, by discharging partially oxygenated water back to the river, ammonia and COD could be improved by the time the water reached Wignehies.
Figure 2. Reoxygenation lagoon for the Helpe Mineure River at Fourmies, France (drawing not to scale).
A sewage treatment plant for the city of Fourmies is located about one kilometer downstream from the site selected for the treatment lagoon. Its effluent contributes additional COD and ammonia to the river. The ideal place to have the site would be at the outlet of the WWTP, but no land was available.
The retention lagoon, Fig. 2, was constructed between Fourmies and the WWTP to hold 12,500 20,000 ml. It is a maximum of 3 m deep, and has a maximum surface area of 7,000 M2. A dam was constructed at the inlet, with a control weir. The river could be diverted to the retention lagoon, or sent directly down the river during floods.
CLEAN-FLO installed eight 2 kw CLEAN-FLO Inversion/Oxygenation systems. Sixty-four microporous ceramic air diffusers were placed evenly at the bottom of the lagoon. No diffusers were placed near the outlet, leaving a small quiescent settling zone. A water pump recirculates 100 1/sec from the quiescent zone back to the beginning of the diffuser area. This reoxygenates 14 – 160 percent of the water a second time, depending on the flow rate. At lowest flow rates of the river, when water quality is worst, 100 percent of the water is recirculated. This pumped water is discharged to the top of a rip-rap stream bed that slopes rapidly down to the lagoon. This adds additional aeration as the water flows down the waterfall back into the lagoon, and the water is reaerated by the diffusers. On January 27, 1992, the system was turned on.
Water quality monitoring equipment was installed at the inlet and outlet of the lagoon. Data was also collected at eight locations from Fourmies 0.12 km upstream from the lagoon, to Rocquigny, Fig. 1 (2). Test point P2 is before the treatment lagoon. P3 is at the outlet of the treatment lagoon and P4, 0.52 km downstream, just before the sewage treatment plant. P8 is 4.4 km downstream from Fourmies at Rocquigny after the river passes through Wignehies (P5, P6 and P7).
Fig. 3 shows elevated and adequate levels of DO from the lagoon to Rocquigny on July 2 8, 1992. Data collected in the aeration lagoon between July 1992 and October 1993 (Fig. 4) show a consistent increase in DO. The magnitude of improvement was generally greatest in the summer during the low flow period. At low flow (May through October), oxygen saturation at 20’C increased in the lagoon from about 20percentto7O-lOOpercent. Although water quality was poor in July 1992, results of the aeration lagoon were beginning to appear. The oxygen level of the river increased (Figs. 3 and 4). Without the reoxygenation lagoon, post-treatment DO from Foumiies to the WWTP would have been at the same level as the pre-treatment levels. As DO varies considerably from day to day, pre-treatment measurements are meaningless.
The Syndicat turned off 20 percent of the aeration equipment from January 4, 1993 to April 15, 1993, to test the efficiency of the equipment. They turned off 50 percent of the equipment from April 19, 1993 to May 7, 1993. During these shutdowns, DO at the outlet was less than DO at the inlet. This test suggested that COD and ammonia were consuming oxygen in the retention lagoon during the approximate 5.5-hr retention time. It also confirmed that the system was not overdesigned.
Flow rate and conductivity of the water coming into the lagoon are shown in Fig. 5. Typical flow in the river is from 150 to 500 1/s. The increase in conductivity during low flow shows that at low flows, incoming pollutants have maximum detrimental effect on water quality. July, August and September are typically the lowest river flow rates of the year. This is when water quality has been the worst at Wignehies and Rocquigny.
On July 28, 1992, six months after the CLEAN-FLO system was initiated, the flow rate was only 73 1/s. It was the worst water quality monitored throughout the test period. Ammonia and COD were still too high at Wignehies (see Fig. 6). This limited data suggested that new pollution was being added to the river between the sewage treatment plant and Wignehies, test points P4 to P5. An investigation revealed that sewage from twenty-five hundred people the river. Water quality was now going directly into in cob and deteriorated in this area. The reduction ammonia between wignehies and rocquigny resulted from both river aeration and the additional DO in the river from the aeration lagoon.
Ammonia and TKN flux were reduced in the retention lagoon, but a greater reduction occurred in the river, Fig. 7. Nitrogen flux in mg/sec is a measure of nitrogen load in milligrams per liter multiplied by flow rate in liters per second. Nitro en flux on July 28, 1992 is compared to nitrogen flux on July 21, 1991 before the lagoon was constructed, Fig. 7. This measurement is independent of differences in flow rate due to rain. Fortunately, both inlet ammonia flux and inlet TKN flux were almost the same on July 28, 1992 as on July 21, 1991. The aeration lagoon reduced ammonia flux 47.5 percent from 400 mg/sec to2lOmg/sec. Total Kjeldahl nitrogen flux declined 48.5 percent from 350 mg/sec to 180 mg/sec.
We hypothesize that ammonia is converted first to nitrite, then to nitrate in the lagoon by nitrifying bacteria in oxygenated water. Then facultative bacteria such as Pseudomonas use nitrate as the electron donor and organic matter as a carbon source to produce nitrogen gas, carbon dioxide and water. This would result in the decrease in TKN approximately equal to the reduction in ammonia.
COD and ammonia classifications improved, Fig. 8. Each value shown in Fig. 8 is the average of three consecutive dates; i.e. the value given for 8/22/ 91 is the average of the classifications on August 21, 22 and 23, etc. Before the aeration was initiated, lagoon influent and effluent have the same values. While there is only a small difference in classifications at the inlet and outlet of the lagoon, a large difference occurred at Rocquigny downstream. Without the reoxygenation from the retention lagoon, classification numbers at Rocquigny should have been approximately the same as those values measured on August 22, 1991 (low flow) and January 13, 1992 (high flow). This difference continued to increase downstream. improvements from Class 2B-3B to Class IB-2, along with increased oxygen and the appearance of fauna improved the total classification of the river from Class 3-4 to Class 1B.
Water quality has remained in Class I to 2 since September 1092, even during a WWTP failure when the aerators were partially turned off during 1993. The increase in COD and ammonia classifications on February 17, 1993 was caused by two circumstances. There was a temporary discharge of ammonia from a breakdown in the sewage treatment plant. Simultaneously, the Syndicat turned off 20 percent of the aeration equipment. This had no lasting deleterious effect on the river. Although it was not a performance goal, Figs. 6, 7 and 8 show ammonia aid COD were reduced in the oxygenation lagoon. Also, in accord with the goals, ammonia and COD in the river were reduced by the oxygenation of the retention lagoon.
One year after the system was turned on, fingerlings about three inches long, vegetation, and diverse benthic organisms appeared in the river from Fourmies to Rocquigny. Fry stocked in the retention pond in the summer of 1992 were six inches long by May 1993.
Two years after abatement of incoming pollutants to the extent possible, water quality of COD and ammonia remained in Class 3-4 at Rocquigny during low flow. During the rainy season, it was Class 2. The aeration project was designed to improve DO, COD and ammonia.
Eight months after the aeration system was turned on, water quality was improved to Class 1B during both low flow and high flow. Fish, benthic organisms, and aquatic vegetation appeared from the aeration lagoon to Rocquigny. This assured that the entire 43 km stretch of the river was improved. With the water oxygenated in the lagoon, continued oxygenation by the river results in a reduction in ammonia and COD downstream. We conclude that the significant difference at Rocquigny between preaeration and post-aeration classifications can be directly attributed to the oxygenation of the retention lagoon. A small improvement in ammonia at the outlet compared to the inlet of the reaeration lagoon was the direct result of aeration. The small improvement in COD in the aeration lagoon may have been the result of a combination of both aeration and settling. These improvements were not significant compared to the greater improvements downstream after reaeration was initiated.
Reductions in ammonia and TKN flux densities on July 28, 1992 (worst post-aeration water quality date) compared to July 21, 199 1, are shown in Fig. 7. The differences continued to increase downstream. Many improvements from pre-aeration pollution abatement were not covered in this paper. Figs. 3,4, 5, 7 and 8 show the improvements after aeration. The establishment of fauna and aquatic vegetation further demonstrates the value of the aeration project. The general results were:
1. The appearance of fish and benthic organisms in both the river and the retention pond.
2. Increase in oxygen saturation from 20 percent to 70-100 percent in the river during low flow, as the water leaves Fourmies.
3 . A consensus of riparian residents is that after aeration, there was a change from an objectionable odor during low flow to no odor, and the water is now much clearer downstream.
4. Reduction of COD, TKN and ammonia for the entire 43 km of river.
5. Improvement in COD and ammonia levels from Class 3-4 to Class 1B. Except for a temporary breakdown in the WWTP and partial shutdown of equipment, Ns water quality was maintained for the entire distance of 43 km downstream, which was unusable before the project began.
In May 1993, the cities of Fourmies and Wignehies were awarded the “Prix de L’Environnement” and “L’Echarpe Bleue” for the improvements in the river. These are the two highest environmental awards in all France.
While incoming pollutants should be reduced to the extent possible, it is often impossible to treat nonpoint source pollutants that enter large bodies of water via rivers. As rivers feed most lakes and reservoirs, river restoration often becomes important to the restoration of a lake.
The authors wish to express their appreciation to David Wright of Minnesota DNR for reviewing the paper and making it more presentable. Many people d to the restoration of the se include first, Mr r Action/Fourmies et Environs for his exhaustive work in coordinating and activating all the people and organizations necessary for the implementation; Fernand Pacheux, Mayor of Fourmies and President of Syndicat Intercommunal D’Assainissement Fourmies/Wignehies; Marcel Dehoux, Deputy Mayor of Wignehies. The number of people who, without their involvement, dedication, engineering design, consulting, and funding of the project, it would not have succeeded, are too many to list here. A few include Claude Houyez, Engineer and Joel Legrand, D.D.E. du Nord, who designed the dam and retention pond; Virgil Muller, P.E., Muller Engineering; Lance Crombie, PhD., Microbiologist; C. Audouin, Director and Jean-Pierre Demacon, Technician, Eau et Force; Michel de Barba, Enterprise de Barba; Martial Grandmoujin, Asst. Director and Alain Herman, Mission Chief, Agence de l’Eau ArtoisPicardie. Many more participated at an equivalent level of dedication and sacrifice who should be listed, and the authors apologize for not including everyone.
Agence de L’Eau Loire-Bretagne. 1990. Description de la Quality des Eaux et Cours D’Eau. France.
Bertrand, J. and C. Christophe, Societe Eau et Force, and M. Journez and M. Karpinski, Amodiag Environment, February 1993. Impact du fonctionment du CLEAN-FLO sur la qualite de L’Helpe Mineure. Interoffice communication. Fourmies, France.