Water & Wastewater Treatment Magazine
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In the know Technically speaking: wastewater treatment sample can be used to retain the observations. These can provide evidence to compare to past samples or information for different analysts completing the examination. The frequency of analysis should be tailored to the site. For the most part, weekly or monthly samples are sufficient for a regular check on the health of the plant. If the analysis is being used to monitor process changes or the commissioning of new equipment, then this frequency might be increased to daily or a few times a week for the duration of time that works are being undertaken. Maintain records of the observations and if these deteriorate then on site plant alterations can be made. For example, if filament Type 021N becomes more prevalent, aeration levels may need to be increased. Microscopy can also be utilised for assessing biomass to be used for reseeding, to ensure this is of good condition prior to delivery to site. Municipal treatment – filament/settlement issues Due to settlement issues on a site with sludge handling facilities, a mixed liquor sample was analysed and revealed that filament Type 021N was present. This particular filament can rapidly proliferate when dissolved oxygen levels are low and when septic conditions are found, as it can metabolise hydrogen sulphide directly. It can lead to poor settlement and at its worst highly viscous mixed liquor, which in turn affects efficient oxygen transfer further exacerbating the low oxygen conditions. Return liquors from sludge handling can be a rich source of the septicity that this filament species can thrive under. If there is insufficient aeration capacity to drive off the septicity and to treat the BOD of the return liquors the activated sludge can rapidly deteriorate. Sludge samples from outlying sites can be days or in the worst case weeks old when they arrive at sludge handling sites. In this case there was a mechanical problem with the on-site sludge handling facilities meaning that when the sludge was being stored on site and processed it was 'old'. Recommendations to eliminate this filament were to process the sludge when it was fresher, have additional aeration available, or as a last resort chemical dosing could be utilised to reduce the length of the filaments, improving settlement. In this circumstance sodium hypochlorite was used to aid the settlement in the short term while the mechanical faults in the sludge handling system were rectified as there was no quick fix solution. Industrial treatment - fungal growth and pH correction Industrial plants have a wide range of problems which can include overloading, toxicity, nutrient deficiencies and foaming. The causes of these can be varied and tend not to be well documented in the literature due to the unique nature of each industry and treatment technologies in use. A recent sample contained a thick mat of biological material which was present on the filter media. Under microscopy examination this was identified as a fungal growth. Fungus tends to grow at pH <6 and when there is a high proportion of easily biodegradable material in the influent. It is able to grow more rapidly than bacteria in cold conditions. They can transfer materials internally more efficiently than bacteria due to the morphology of the fungus and therefore are particularly able to exploit low oxygen and nutrient conditions. The observation of the fungus in the sample meant that the site was able to check on the upstream pH balancing which was not performing correctly; this was rectified removing the conditions that had allowed the fungus to proliferate. The plant still contained a large amount of fungal matter which needed to be removed from the filter bed. Advice was given on how best to achieve this which included backwashing at the maximal rate to break the fungal filaments and then to wash through into the backflush system for removal for offsite disposal. Conclusion These examples show that microscopic examination is a beneficial tool which can be used to pinpoint the causes of treatment problems. The information gathered can then be used to ensure that the final effluent consistently achieves the required consent limits using process changes and engineered solutions, ahead of expensive chemical dosing, which may fall under the scrutiny of the regulatory authorities. A fungal mass on the filter bed can be a sign that upfront pH correction is not working correctly • Problems and their microscopic causes ● Poor settlement and stringy mixed liquor: A prevalence of filament Type 021N can be caused by the introduction of septicity from sludge handling facilities. When on-site sludge handling facilities experience problems, sludge can be old when it is processed and the return liquors high in sulphides as a result. ● Foaming on activated sludge plant and on secondary clarifiers: This can be caused by Microthrix parvicella, a filament that continues to be present at problematic levels in a lot of municipal activated sludge plants during the winter but tends to disappear during the warmer summer months. ● A thick mat of material found growing in filter bed: Examination of the material will o en show it to be a fungus, caused by upfront pH correction not working correctly. Overhauling the pH correction should remedy the problem. 38 | JUNE 2015 | WWT | www.wwtonline.co.uk