SINIESTRALIDAD DEL SECTOR

Microbial populations monitoring in this system would be contributed to by an additional, intensive sampling effort (alongside further physico-chemical

assessment), which would be performed in an effort to correlate culture-based data with molecular microbiological information.

Such information could be manipulated to produce a calibration tool, linking MPN counts to standard changes in community composition, as assessed by DGGE gels. This would be used as an informative tool, for troubleshooting and analysis of poor system performance, as well as providing further knowledge regarding bacterial community composition in package systems.

Further interrogation of bacterial communities contributing to nitrification,

denitrification and phosphorus removal is required in particular. The presence and activity of ammonia-oxidising archaea and anammox bacteria is of great interest to the academic community, and also has implications for the operation of wastewater treatment systems. The anammox process, in particular, has shown great

efficiencies in the treatment of high nitrogen, low carbon wastes (Third et al., 2001). While this does not describe the nature of waste entering Petersfield WTW, it suggests that, should anammox communities be found in the BESST, this

technology can be adjusted and applied to the treatment of different types of waste, for example, landfill leachate.

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The presence and activity of denitrifying communities in the system can be more effectively analysed by first examining the presence and function of denitrification enzymes. This could be assessed in the BESST using activity assays, such as acetylene reduction techniques (Williams, 1993), or by using molecular techniques to identify the presence (by examination of extracted DNA) and function (by examination of extracted RNA and the transcriptome of the system) of specific bacterial genes, coding for denitrification enzymes.

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7. Conclusions

1. The BESST is able to reduce concentrations of ammoniacal nitrogen,

biochemical oxygen demand and total suspended solids in municipal wastewater to an industry standard level. Over the course of the sampling period, assigned

consent levels were not met consistently, and this was due to problems arising with mechanical failure, experimental disruption and unforeseen circumstance during treatment.

2. The nature of the influent supplied by Petersfield Wastewater Treatment Works was highly variable with regard to influent concentrations of ammoniacal nitrogen, biochemical oxygen demand and total suspended solids, which contributed to the difficulties experienced in establishing experimental changes to the system. Influent variability contributed to effluent variability, and therefore final effluent quality. Influent concentrations of BOD and TSS were high compared to average UK wastewater concentrations. This was due to the time of day during which samples were taken, and possible contribution from sedimentation around the pump delivering waste to the BESST from the top of the WTW.

3. Each area sampled from the system, the anoxic and aerobic zones, the clarifier, the influent and effluent, demonstrated a physico-chemical and microbial profile which was distinct from all other areas, apart from when the system was

undergoing failure due to washout. This indicates that all areas of the system contribute in a different way to overall treatment efficiency.

4. Removal of TSS from the influent was performed by the clarifier, which was comprised of three areas defined by different physico-chemical characterstics – the

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bottom, middle and top. The angle of the clarifier installed in the Petersfield BESST was not suitable for this system, and contributed to the development of too high a sludge blanket. This was subsequently corrected for the design of newly commissioned systems.

5. The DO and MLSS in the aerobic zone can be used as sewage control parameters for this system, but because of the sensitivity of the system to change, optimal performance would be achieved by a computer controlled-feedback system, used to adjust RAS rates in response to concentrations of DO in the aerobic zone. This would reduce the frequency of direct operator contact.

6. The microbial population in the BESST was diverse, complex and changed in response to aspects of the system such as SRT. There was also clear indication of reduction in bacterial populations between the influent and the effluent, observed in DGGE gels.

7. Evidence suggests that nitrification in the system was performed by functional groups of bacteria. However, it was not possible to identify any specific groups performing this function. The majority of the levels of total oxidised nitrogen (TON) observed in the BESST were present as a result of nitrification, and a larger proportion of TON was present as nitrate (NO3-). Populations of bacteria

performing nitrification were a robust community, and maintained activity through most disruptive events. However, dilution of biomass in the system as a result of high rainfall and weak influent (i.e. when the system was fed with primary effluent), led to nitrification failure.

8. The culture based methods used for the enumeration of bacteria worked well as a comparative tool between samples. Used in combination with regular, general

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qualitative analysis of the system and physico-chemical assessments an invaluable familiarity with the system is established, which is necessary for the successful operation of one so sensitive.

9. Molecular analysis of a system such as this is impractical for regular monitoring as part of a maintenance package, but is an effective resolving tool for the

investigation of microbial community composition in the BESST. The practicality of techniques such as DGGE may be enhanced by the development of techniques to correlate molecular data with that obtained from culture based methodology. For example, by the application of a high frequency, sample intensive effort, it may be possible to correlate information obtained from MPN counts with DGGE analysis, and therefore produce a package of data for ‘calibration’ of MPN counts

subsequently obtained without parallel molecular analyses.

10. Specific functional communities of bacteria found in this system require further analysis, using higher resolution tools, in particular, DGGE and FISH. These tools are relatively low cost, but require expenditure of time for optimisation purposes and specific knowledge of molecular microbiological techniques.

11. Failure of the pump delivering influent to the system resulted in either the short accumulation of large levels of solids in the system with a clean effluent, or the accumulation of solids with a very poor effluent. In this instance, poor effluent was also associated with the unusual presence of large quantities of grease, which exacerbated the problems already caused by pump failure. Accumulation of solids as a result of pump failure was attributed to the accumulation of sludge, as a result of dead biomass.

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12. Air flow failure did not lead to unmanageable problems in the short term, but prolonged failure led to similar effects on the system as those seen for main pump failure. This was attributed to accumulation of sludge as a consequence of dead biomass.

13. The dosing of acetic acid into the anoxic zone led to luxury uptake of polyphosphate, by filamentous populations which increased in quantity as a consequence of the additional carbon source. Therefore, the observed effect on phosphorus removal, which was positive overall, was associated with large

quantities of bulking and foaming in the system, which led to operational difficulty, and was a difficult issue to solve.

14. The adjustments to SRT did not produce conclusive results due to unrelated problems occurring at the same time as they were made. However, the drastic reduction of SRT instantly led to a stall in nitrification.

15. It was not possible to draw conclusions regarding the effect of cold weather on the BESST. Disruption to treatment which occurred at the same time as winter periods was likely to have been responsible for the poor treatment observed. 16. The application of multistage treatment processes to a package system presents a number of difficulties, all of which relate to its size. There is less room for error regarding operation control parameters, loading, and toxic occurrences. Because the area of biomass in the system is small, there is no ‘buffer’ zone which can

accommodate problems such as these. However, the system displayed a degree of robustness, which suggests that once proper protocols are determined and adhered to, the BESST is a viable and resilient option for the treatment of a smaller waste flow. Such protocols can be established by an initial sustained sampling effort,

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which would monitor physico-chemical and microbial variables to assess steady state characteristics and therefore suggest optimum control procedures.

17. A dataset as large and varied as this requires extensive multivariate analyses and modelling, in order to realise the full potential of a study such as this. In this way, an initial sustained monitoring effort would contribute positively to the lifelong operation of a system. Employing a range of multidisciplinary techniques leads to the generation of a large amount of multi-faceted data, which, when suitably analysed and modeled, can be used to solve problems and enhance treatment performance.