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Tackling low F:M ratio and high OD

There is no ideal Food to Microorganism (F:M) ratio that will work for all activated sludge treatment systems as the wastewater treatment system varies from place to place with unique ideal or optimal F:M ratio. The best F:M ratio for a particular system depends on the type of activated sludge process and the characteristics of the wastewater entering the system. The F:M ratio of a wastewater plant can be calculated daily with COD and mixed liquor suspended solids (MLSS) data. This value then can be correlated with effluent COD and TSS values to determine the optimal F:M ratio range. Francesco Ortepice, a civil engineer from Milan has raised a practical scenario of having low F:M ratio and high OD in aeration basin. Let’s read the expert’s view and their suggestions on how to tackle the issue.

Francesco: There is a situation: A plant of capacity 20m3/h with TSS into the final effluent, from the secondary clarifier could be due to the Low F:M ratio and the high OD (6mg/l) in aeration basin. There aren’t filamentous bacteria and the Sludge Volume Index (SVI) is about 150. What are the possible problems that could arise in this case and how much time will it take to solve the problems with reference to the Solids Retention Time (SRT) – aka: sludge age?

Rick Fuller, Senior Technical Advisor-Athlon Solutions: Given the case, low F:M ratio but a high OD of 6mg/lt, (oxygen demand and not dissolved oxygen), the typical COD/BOD ratio is approximately 2.8. If it is dissolved oxygen concentration in the bioreactor, it implies that with little food coming in the oxygen demand is actually quite low. A “so-called” normal value for the COD/BOD ratio is 2.1 and the last check indicates this ratio to be 2.2. As the COD/BOD ratio increases above 2.1, one can anticipate that the bugs are dealing with a higher concentration of recalcitrant or hard-to-degrade organic compounds. Currently, compared to the average COD/BOD ratio, it would appear that the food coming into biological treatment is more amenable to oxidation rather than less. That is, the food that is coming in appears to be readily available to microorganisms.

So, what is the issue? Over-aeration definitely seems a possibility and filamentous growth could be a possibility too. In such a scenario, one must try to have at least 1mg/lt of phosphorus available in the clarifier overflow to ensure a sufficient supply of this nutrient to the biomass. To confirm nitrification the pH of biological system inlet and clarifier outlet must be measured for nitrate, ammonia, and alkalinity. Nitrifying the pH, alkalinity and ammonia will drop and the nitrate will increase from the influent to the effluent. It is also carrying many solids in the clarifier based on the sludge blanket numbers provided. If the sludge is dispersed throughout the water column which is likely, given the solids carryover issues, some polymer addition (a cationic flocculant) to the clarifier inlet can be considered to help solids settle and improve the removal rate of these solids from the clarifier.

Michael Nash, Kitchen showroom/Instalation manager: I would never advise using polymer to aid settling as it blankets the biomass resulting in anaerobic conditions resulting in a dead aerobic plant! It is all about balance and information that is key to a lot of testing on all parameters and ensuring nutrients and micronutrient levels present and more importantly bioavailable. Here, it is worth noting that Superphos generally not bioavailable.

Rick: My experience with polymer chemistry, quite in contrast to Michael’s, has been, without exception over the last 35 years, very positive.

The solids in a wastewater plant, including the mixed liquor suspended solids that need to be settled in a secondary clarifier, have a negative surface charge. So to be more precise, in recommending polymer as a settling aid I say this: You need to use a cationic flocculants diluted to a 1% solution, or less, and your polymer dosage rate would be in the range of 1 to 12ppm. Given such a low dosage of a synthetic organic polymer you will have no risk whatsoever of blanketing the biomass.

If your wastewater flow consists primarily of municipal wastewater you should rarely have any need to use polymer to help solids settle in the clarifier. But as your contribution from industrial wastewater increases, you may find, perhaps regularly or not, that your biomass is “stressed”. This can happen due to temperature changes, pH changes, inhibitory or refractory compounds, nutrient deficiency, etc. The problems can be numerous and quite varied. When this happens, you will find that a “settling aid,” a polymer, can provide an excellent solution to reducing dispersed solids in the clarifier. The polymer chemistries available to us today are, simply stated, remarkable. You can find a wide range of products customized to provide the exact degree of crosslinking and charge to suit your specific needs. All you need to do is contact a polymer vendor and they won’t hesitate to send someone to your plant to do jar testing where you can directly see for yourself the benefit of a chemical program to help in time of need.

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