Hitting ammonia consent levels
Mike Pearce of Envitech discusses how new sensing technologies and reconsidering strategies can help water companies meet tighter consents in the field of nutrient control
The increasing legislation, both from Europe - Water Framework Directive (WFD) and Urban Waste Water Treatment Directive (UWWTD) - and within the UK - Integrated Pollution Prevention Control (IPPC) - has resulted in greater stringency when granting discharge licences, in particular in the field of nutrient control. Ammonia consent levels are reducing significantly and the number of discharges subject to phosphate consents are set to approximately double over the next five years.
This, taken in conjunction with the limitations being imposed on discharge of aluminium or iron to watercourses, has forced water companies to reconsider their strategies for phosphate removal. Where in the past biological phosphorous (P) removal has been passed over in favour of chemical treatment, this option is having to be reassessed. Biological nitrogen removal (BNR) is now almost taken for granted, with ammonia oxidation and nitrate reduction being performed in dedicated anoxic and aerobic process zones. However, the complexity and stringency of required process conditions, when an anaerobic zone for P release is required, increases significantly. Consider just one process variant of the BNR system as shown in Figure 1 for the University of Cape Town (UCT) configuration.
There are three extra flow streams, all of which may have variable volumetric rates, which in turn will affect critical parameters in the zones to which they are discharged. It is essential the anaerobic zone does not contain any significant levels of NO3 or DO, otherwise P release will not occur. Similarly, it is necessary to operate the anaerobic zone within certain concentration limits for volatile fatty acids (VFAs) otherwise the polyp bacteria will not be favoured. The success of the process depends on sufficient P release in the anaerobic zone, hence it would seem prudent to know what this concentration is.
Recirculation into the anoxic zone must be controlled so that dissovled oxygen (DO) levels are kept low, similarly the rate of return should not exceed the capacity of the zone to denitrify completely, otherwise NO3 would be introduced into the anaerobic zone, therefore nitrate levels should be monitored at the outlet to anoxic zone. Nitrification of the ammonia in the aerobic zone is critically affected by the level of MLSS and the concentration of dissolved oxygen. It is fairly obvious that to be able to operate such a plant and stay in control requires a much higher level of parameter knowledge than hitherto needed.
Not only that but these parameters need to be relevant to the position in the process and available in real time. Hence reliable, insitu parameter measurement with minimal maintenance requirement is a prerequisite of satisfactory performance. These requirements have driven instrument manufacturers to both adapt existing technologies and to introduce new technologies to overcome the difficulties presented. Probably the best known activated sludge parameter currently measured and used online is that of DO. Up until recently the technology used has changed little over the past 20 years, being an application of polarographic cells in conjunction with semipermeable membranes. In its time this was a significant breakthrough but operating experience has shown it not to be without its difficulties. Fouling, calibration drift, accidental rupture of the delicate membrane, limited accuracy at low DO levels and high maintenance requirements have all led to a new search for the Holy Grail of DO measurement. This has led workers at InsiteIG to develop a technique used extensively in the medical field, that of fluorescence technology. The principle is fairly simple - a ruthenium compound, immobilised within a sol-gel matrix, is internally irradiated by a 475µ light source. This excites the outer electrons in the ruthenium complex to enter a higher energy level. When they collapse back into their original orbit they emit energy at 600nm, for example, they fluoresce.
Provided the transmitted light is tightly controlled, the degree of fluorescence is both predictable and repeatable. In addition, the degree of fluorescence is directly affected by the presence of oxygen molecules - it is reduced or quenched. It is this characteristic that has allowed the fluorescence to be used for DO measurement. The advantages of this system are:
Other similar techniques are available using luminesence rather than fluoresence, although some of these sensors are significantly degraded when exposed to ambient light, even for a short period. It is for this reason they still have to have replacement gel caps at least once a year. This is not the case with the InsiteIG probe. These developments now permit low-maintenance, high-accuracy implementation of DO measurement, even in the activated sludge zones not previously measured. As illustrated in the UCT flow chart (Figure 1) it is desirable to have the ability to measure nitrate, organic strength, MLSS and settleability, all within the process. Individual instruments for these parameters have been available for a while, however, one manufacturer has brought out a new approach by combining all these measurements into one single sensor, the StipScan from Isco Stip. This device is a broad band scanning spectrophotometer, which operates in-situ within the activated sludge. There are no chemicals used and there is only one moving part, the piston, used to draw a sample into the measuring chamber, making the unit is virtually maintenance-free. When the sample first enters the quartz cylinder the absorption at the infra-red end of the spectrum is used to measure the suspended solids.
The sample is then allowed to settle and the absorption spectra logged with time. The measured slope is directly related to the settleability and sludge volume. The software analyses the settling curve and dictates when to take the readings for the nitrate and TOC/COD bandwidths, removing any solids interference. Mathematical algorithms are then employed to separate out the overlap of the nitrate and organic content absorption spectra, giving improved accuracy on both measurements. The Stipscan is thus a major step forward in providing multiple critical parameter measurements for activated sludge control in one simple, maintenance-free package. It furnishes the user with MLSS, nitrate, COD/TOC, settleability index and sludge volume. The remaining parameters of interest are phosphate and ammonia, for which there are well-established in-situ measurement probes. There are more than 100 Stip ammonia process buoys in operation within the UK, both in final effluent and mixed liquor applications, and there is a growing interest in the potential for dynamic aeration control in the aerobic zone using the ammonia measurement. Tests have been performed both in SBR and plug flow applications using a Stip ammonia process buoy to control the aeration time in order to achieve the desired ammonia level at minimum cost and energy consumption.
The indications are that energy savings of 10-15% are achievable in winter and possibly greater in the summer, which on medium-large plants would give significant financial savings and short payback periods for the equipment needed. An added benefit is that the effluent quality is being continuously monitored prior to discharge, thus satisfying the regulator's demands that the discharger should self monitor. In brief, wastewater monitoring has come a long way in the last few years and we are now on the verge of the next big change. That is, dynamic control using multiple parameter monitoring to achieve higher effluent quality and reduced energy consumption with minimum maintenance requirement.