Technically Speaking: Sludge under the microscope
Microscopic examination of sludge or biomass can reveal the biological roots of common problems within a treatment process
by Emma Morley, Senior Process Scientist, Aqua Enviro
The activated sludge process may have been in existence for 100 years but that does not mean that it operates without problems. The same is true of all biological treatment processes, whether they are a new innovative design or a more traditional trickling filter.
Microscopic examination of activated sludge or fixed film biomass is a technique that can be used to identify the causes of poor settlement, foaming and other operational problems, the outcome of which can threaten compliance with consent limits. After identifying the microbiological cause, actions can be taken to eliminate the problem.
A wide range of pressures apply within the municipal and industrial sectors. Many of these are the same across both types of wastewater treatment plants – such as pressure from regulators and economic drivers to be more efficient in terms of energy, chemical and manpower levels. Municipal plants are at the mercy of new developments, the impacts of climate change (such as dealing with rainfall and flooding on a more frequent basis) and on-site sludge handling issues. Industrial plants can have the added challenge of the current wastewater having altered significantly from when the effluent plant was designed.
All of these may lead to small (or large) changes in the way an effluent treatment plant is operated and can affect the delicate balance of the microbiome present within the treatment system. This could lead to poor settlement and foaming in an activated sludge plant and under or over growth of a biofilm in an attached growth process. The outcome of these could lead to consent limits being threatened due to solids carryover or incomplete treatment.
Microscopic examination of the mixed liquor or biofilm can be used to identify the biotic causes of many operational issues. Once the microbiological reason has been established, actions can be taken to eliminate the underlying causes. These could be simple things like introducing or increasing balancing volume or increasing aeration capacity. On the other hand, large-scale capital schemes for plant upgrades may be required.
Methodology for microscopic examination
Samples of biomass are collected from the effluent treatment plant; this may be mixed liquor or scrapings from a fixed film process such as a trickling filter or a SAF media. The samples should be examined soon after collection, especially for high food-to-mass (F/M), low sludge-age plants.
Key observations should be made of the floc structure, protozoal and metazoal populations, filamentous bacteria (Type and abundance/length), and the clarity of the bulk liquid (space between the flocs). Filamentous identification to a type or species level is achieved with the use of Neisser, Gram and Indian ink staining where appropriate. Using these characteristics it is possible to assess the sludge age and the F/M, identify signs of insufficient oxygen and deficiencies of other nutrients as well as noting that shock conditions may have occurred. Shocks might include rapid changes in influent pH and BOD concentration, presence of chemicals or detergents from spillages and changes in temperature. The exact nature of these can be later verified through site knowledge and operating data.
If possible photographs of the 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.
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.
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