Sludge Wasting: An important tool for Activated sludge performance optimization
Conventional Activated Sludge process, one of the most widely used technology for the biological wastewater treatment has undergone a long cycle of testing, research, development. The technology has been well studied over the decades for different applications. One of the striking features of the technology is its simplicity of operation and control provided the operator understands the key operating parameters and its control.
Indeed there are many process parameters like Organic loading rates, Dissolved Oxygen Concentration, Sludge Volume Index (SVI), Food to Mass Ratio (F/M), Oxygen Uptake Rate(OUR), MLSS to MLVSS ratio , RAS flow are being used by operators for its control and maintenance of correct operating conditions. But one specific parameter which stands out among all is Sludge wasting. Yes, wasting of excess biomass or MLSS from the activated sludge is perhaps the most important parameter which most of the plant operators are not taking seriously due to many reasons. Maybe they have no space or treatment capability available to handle the excess sludge or may be the plant owner is not happy to handle extra cost of sludge management and thus discouraging the wastage of sludge from the system.
But wasting sludge daily in correct proportion and keeping the MLSS refreshed all the time is as vital as providing right amount of air to the aeration basins. During our assessment and survey of some of the activated sludge process for industrial as well as municipal wastewater treatment, we found that most of the plants were running without formal sludge wasting. Many of the plants had not wasted sludge nearly for two years. Most of the operators were under impression that higher the MLSS, better would be removal efficiency and performance. They kept telling me that a lot sludge settle here and there within their system that they never needed sludge wasting formally. It was shocking revelation to us that most of the plants had been performing poor and having sub optimal biomass growth with very poor oxygen uptake rate capability. Imagine a condition where we keep feeding our stomach with new food everyday but never go for faecal discharge then what happens! Same thing is true about wasting sludge in activated sludge process.
Some of the serious consequences of not wasting sludge on regular and continuous basis are as below:
Accumulation of Inorganic and toxic pollutants and inert solids within the MLSS
It is common phenomenon in activated sludge that MLSS or floc particles act as binding agent or adsorptive media for some of the inorganic and organic substances present in the incoming wastewater. In fact, for most of the organic substances which otherwise nonbiodegradable, their adsorption or capturing in the MLSS floc and removal via wasted sludge is the main mechanism of their removal from liquid stream. If there is no wastage of sludge at all then slowly there would be steady build up of these pollutants in the MLSS. Inorganic pollutants will reduce the MLSS to MLVSS ratio and thus slowly actual removal efficiency of the biomass would reduce. If the organic pollutants captured are toxic and nonbiodegradable then once their threshold values are crossed, it will start imposing toxicity on the microorganisms growing in the activated sludge and will reduce their growth and subsequent removal.
For many industrial activated sludge processes, a lot of chemicals are added in the primary treatment and there is always a steady of inflow of these chemicals to the aeration basin and become part of the MLSS present in the aeration basin. When there is no wasting then steadily their concentration will build up to toxic levels, change the charge of the MLSS and thus reducing its settling properties. In worse cases, the specific gravity of the MLSS increases to such level and it becomes so heavy that wherever there is slight dead zone in the aeration basin, the sludge will start settling at the bottom of the tank and with this settling, active biomass will also settle at the bottom of the tank and will be lost forever.
It is also detrimental to the diffused aeration systems. We have observed at one plant that their collapsed diffusers were totally coated with lime deposits because the client was using lime heavily for their primary treatment and sludge was never wasted in this plant nearly for four years. So the heavy accumulation of lime had caused precipitation in the aeration tank causing scaling of lime on the diffusers.
So not wasting sludge has serious consequences as it directly impacts the performance of the activated sludge process.
Extremely low Food to Mass(F/M) ratio and associated problems
When there is not formal sludge wasting, steadily the Food to Mass ratio commonly known as F/M ratio gets increased. F/M ratio is an operating parameter which indicates us the probable microbial population dominating the MLSS fraction. Under extremely low F/M ratio conditions, the amount of food available to microorganism gets reduced and thus their growth rate as well as over all activity gets reduced. Under low F/M ratio conditions, many of the filamentous organisms start dominating the microbial population reducing the settling index and settling properties of the sludge. Very low F/M ratio develops conditions favourable to predator organisms which survive on eating live as well as dead biomass. In worse case conditions, deflocculation of the floc particles will happen increasing the turbidity of the effluent due to poor settling of these fine floc particles getting carried over with the treated effluent. This fine floc particles also increase the BOD and COD values of final treated effluent reducing overall removal efficiencies of the plant. Another issue with very low F/M ratio condition is the toxic shock load capability of the biomass. As due to above mentioned reason when the actual biomass activity is reduced, under toxic shock load conditions, the already sluggish, slowly growing biomass will not be able to cope up the toxic shock load and may succumb to it causing severe death rate which further reduce the performance.
So wasting excess biomass or MLSS is one of the most important operating parameter and aspect of maintaining good and healthy activated sludge process. But again, a big question comes in one’s mind, how to waste sludge and what is the way out???
Stay tuned with us and in our subsequent blogs, we will discuss various methodology of sludge wasting and monitoring your activated sludge process well enough.