Airport surface water runoff: applications for on-line BOD analysis Jim Pickering*, Colin Genner* and Patrick Keller** * Envitech Ltd., Lambourne Crescent, Llanishen, Cardiff CF14 5GF, Great Britain (E-mail: [email protected]) ** Stip Isco GmbH, Siemensstr. 2, 64823 Groß-Umstadt, Germany (E-mail: [email protected]) Abstract The extensive surface area of airport runways generates large volumes of runoff water which depending on the time of year can be heavily polluted from organics. Typically on-line TOC analysers had been used for this type of application but this paper will describe how at Gatwick Airport, London an on-line BOD analyser was tested successfully and installed. Following on from the Gatwick installation, similar on-line BOD monitoring systems have now been installed at Manchester, Birmingham, Stansted, East Midlands, Aberdeen and most recently Heathrow airports. Furthermore, success with similar applications in Germany and the US has been achieved. Keywords BOD on-line method, deicants, kerosene, runoff water INTRODUCTION This specific field of application for BOD on-line analysis started at an airport in England in the year 1995. The airport had to monitor the surface runoff water continuously. Depending on the level of organic pollution, the surface runoff water is allowed to go direct into the local river, or must be diverted to the local municipal waste water treatment plant. BOD was the parameter of choice, primarily because the consent limit was expressed in BOD, but also the airport had poor experience with TOC for the very same application. This paper describes the installation and the results of STIP ISCO‘s BOD on-line analyser BIOX 1010 at Gatwick airport, London. EXPERIMENTAL The application Typically, the airport surface runoff waters can contain organic pollution in form of kerosene/oil and, in winter during the deicing period, deicants such as glycol and konsin. Thus the organic load and the matrix can vary greatly. Dependent on the actual organic load the surface runoff water must be treated or can be send directly to the river, lake, or sewer system. The treatment of this runoff is usually done in a local waste water treatment plant. In contrary to industrial waste water treatment plants these are very often not equipped to handle high swings of organic loads and varying composition. This situation requires careful monitoring of the runoff water in order to divert for treatment or allow direct discharge. Furthermore, very importantly, on-line monitoring can help to reduce the volume of surface water that has to be held by the airport since rapid measurement can allow release much sooner than relying on laboratory analysis. Operating such an on-line analyser as part of an automatic control system can result in significant cost savings in terms of size of holding tank required and allowing much greater control of existing holding capacity. In addition on-line analysis can also protect a treatment plants biomass and aids effective treatment. The delay of 5 days for laboratory BOD analysis resulted in the need of a rapid method and hence the BOD-M3 on-line BOD method was chosen to obtain results, which mirror the present situation with a short time delay. Therefore, the STIP ISCO analyser BIOX 1010 was chosen. The BOD-M3 on-line BOD method The continuous short-time BOD-M3 measurement technique incorporates a procedure where microorganisms control the computer of the monitor [1]. The effluent sample is continuously pumped to the instrument where it is coarsely filtered and then diluted by an automatic feedback control system so that the internal immobilised biomass is fed with a constant level of waste sample. The required dilution ratio of waste to dilution water is then used to calculate the BOD. Waste sample concentration within the bioreactor is kept constant and at a low level to achieve a constant 3mg/litre oxygen consumption by the microbes. Thousands of small plastic rings within the bioreactor provide a growth surface for the biomass which is in constant contact with the waste sample and so is adapted to variations in the waste sample in the same manner as a biological waste treatment system. Toxic shocks are rarely a problem due to the feedback dilution control system. A key factor of the success of the BIOX BOD analyser is the large flows and large diameter pipes for analysis. Typically a BIOX will be analysing 1000 times the volume measured by a standard TOC monitor. This helps greatly to reduce any problems with blockages and ensures a true representative sample is measured. In addition a simple sample preparation technique i.e. coarse filtration with a 0.5mm mesh size with built-in autoflushing enables the BIOX-1010 to operate almost without the need of maintenance. Fig. 1: The BOD-M3 analyser BIOX 1010 for on-line monitoring The measuring site’s specific factors The BIOX requires a continuous source of organic material and nutrient in order for the maintenance of the growth of the biomass. Typically this is supplied from the analysed sample and hence there are no reagent requirements for the BIOX analyser. However in the case of an airport application it was considered possible that at periods during the summer very low levels of BOD/nutrients could be encountered. Hence tests were done to operate the BIOX sampling using simply tap water and with an external dosing system to provide sufficient BOD and nutrients to maintain growth. This was tested at Envitech's site. RESULTS Laboratory experiments During the first tests in the laboratory, artificially prepared samples were measured using the BODM3 as well as the standard BOD5 method. The obtained results point to a strong correlation of the online BOD-M3 values to the real concentration of the sample. In contrary, the standard method showed significant deviations particularly at high glycol concentrations. These type of results are not unusual due to the inherent variables encountered within the laboratory BOD test. BOD 250 Glycol BOD-5 BOD-m3 200 150 100 50 0 1 2 3 4 5 6 7 8 9 Sample No 10 11 Fig. 2: Comparison of BOD5 and BOD-M3 at various concentrations The results from the BOD-M3 test gave promising results. They give a clear picture about the real glycol concentration over a wide range. This confirmed the approach to install the Biox 1010 at Gatwick Airport. During the winter season, high glycol concentrations were to be expected. On-site installation At the very beginning of the installation in 1996 it was found that the BOD/nutrient levels were always sufficient to maintain the biomass and so a control dosing system was not required. At other sites this may be required but Envitech's tests did show such a system can be implemented without any significant problems. Low BOD concentrations were confirmed during the summer 1997. Between June and November mainly values between 20-30 mg/l were obtained. The main source therefore was likely to be kerosene. After the start of the deicing season the BOD values increased significantly to 150-250 mg/l BOD. The strong variations during the winter are based on dilution effects of the glycol containing de-iceant. If there are heavy rainfall or snow events, the deicing component obviously is diluted and lower values are observed. High values were achieved during cold weather and low dilution by snow or water. As observed in the laboratory tests, the good correlation of the on-line and the standard procedure at low concentration was confirmed. At higher BOD-levels greater deviation is evident. A complete record of the BOD measurements between May and December 1997 is presented in figure 3. 300 250 200 Lab BO D 150 BIO X BO D 100 50 0 date Fig. 3: Comparison of BOD5 and BOD-M3 at Gatwick Airport As shown, the installation of a STIP ISCO analyser gave reliable results compared with the BOD standard method. This is one factor to consider in deciding how to measure the runoff water. Other important criteria are the response time and the reliability of the analyser. During the first years of this operation the analyser has shown that overall 99% availability was realistically achievable. The response time of the analyser is described below. BOD mg/l 400 300 200 100 23:20 22:10 21:00 19:50 18:40 17:30 16:20 15:10 14:00 12:50 11:40 10:30 09:20 08:10 07:00 05:50 04:40 03:30 02:20 01:10 00:00 0 Fig. 4: Daily curve of a BOD-M3 at Gatwick Airport in November At Gatwick after deicing the aircraft and the runway the runoff water flows by gravity to a large holding pond. Under level control the runoff water is then pumped up to a balance tank where the BIOX-1010 analyser takes its sample. The peak in the trace occurred as the large pumps were switched on and resulted in a rapid concentration change within the balance tank. This was rapidly detected by the BIOX analyser and hence gives a clear test of the speed of response of the complete analyser and sample handling system. Hence at Gatwick the BIOX-1010 has been used for 5 years as part of an automatic divert control system which regulates the concentration of BOD that is diverted either for direct discharge or for treatment at the local waste water treatment plant. Operational experience Realistically, the maintenance requirements have been between 30 minutes and one hour per week maximum, with several weeks without any intervention at all. Typical maintenance activities are simple things, such as cleaning an oxygen sensor, calibrate a peristaltic pump, and change tubes. Since these simple procedures are under direction of the integrated process computer, they can be successfully done even by non-trained personnel. In addition Envitech provides a remote monitoring maintenance facility whereby the operation of the analyzer is checked using access via a modem connection. This also reduces both downtime and running costs. Realistic total yearly running costs for a STIP Biox-1010 BOD analyser for the described applications amount to approximately 600 US dollars, for 24h/day, 7 days/week continuous operation. SUMMARY It was shown that the STIP ISCO analyser BIOX 1010 meets the requirements for airport surface water runoff. High reliability, accuracy, low cost of operation and low maintenance point to a great benefit [2,3, 4]. The environment as well as the biomass within the waste water treatment plant are significantly protected using on-line analysers for continuous monitoring. Based on the success of this first airport BOD application at Gatwick, several STIP analysers for BOD have been installed for the same or very similar applications. At Gatwick itself an additional 4 analysers are planned for control at the local waste water treatment plant as well part of the environmental control at a new fire fighting training facility. To date there are twenty STIP BOD analysers in six British airports, and two in two German airports. Several US airports have shown significant interest, and so has the AAA (American Airline association). First installations in the US have now been completed. It is anticipated to have additional applications very soon. REFERENCES 1 Köhne, M., Siepmann, F. W., and te Heesen, D. (1986), A Comparison of the BOD5 and the Continuous Short-Time BOD (BOD-M3), Korrespondenz Abwasser, No 33 pp. 787-793. 2 Minting, P. (2001). Technically speaking. In: Water and Waste Treatment Journal; May 2001, p.29. 3 Brown, M. (2001). Here comes BOD. In: Environment Business Magazine, March 2001, p.41 4 Pickering J. (2001). International Environmental Technology, Nov. 1999, Issue 9, pp. 17-18.