What is the key difference between conventional and modified activated sludge processes?

The key difference between conventional and modified activated sludge processes lies in how they manage the microorganisms responsible for wastewater treatment, specifically concerning the presence and behavior of filamentous bacteria. Both processes utilize activated sludge, which is a suspension of microorganisms – primarily bacteria, but also protozoa and other small organisms – that consume organic pollutants in wastewater. This process occurs within a reactor called an aeration tank, where air is supplied to provide oxygen for the microorganisms to break down the waste. Following the aeration tank, the mixed liquor (wastewater and microorganisms) flows to a secondary clarifier, where the microorganisms settle out as sludge, which is then recycled back to the aeration tank to maintain a high microbial population.

Conventional activated sludge (CAS) is a relatively simple process. It relies on a balanced microbial community within the aeration tank. This balance means that floc-forming bacteria (bacteria that clump together to form larger, easily settleable flocs) dominate, alongside protozoa that graze on free-swimming bacteria, further contributing to a stable system. However, under certain conditions – such as low oxygen levels, high sulfide concentrations, or specific ratios of food to microorganisms – filamentous bacteria can proliferate. Filamentous bacteria do not form good flocs; instead, they create long, stringy structures that prevent proper settling in the secondary clarifier, leading to a phenomenon called 'bulking sludge'. This results in poor effluent quality and operational problems.

Modified activated sludge processes are designed to specifically control and minimize the growth of filamentous bacteria. Several modifications exist, but they all share the common goal of disrupting the conditions that favor filamentous growth. One common modification is the extended aeration process. This involves significantly increasing the aeration time (and therefore the hydraulic retention time – the average time wastewater spends in the aeration tank) and reducing the food-to-microorganism ratio (F/M ratio). The longer aeration time allows for the complete oxidation of organic matter, reducing the availability of readily biodegradable substrates that filamentous bacteria thrive on. The lower F/M ratio reduces the competition for these substrates, further inhibiting filamentous growth. Another modification is the sequencing batch reactor (SBR) process. SBRs operate in batches, cycling through fill, react, settle, and decant phases. This allows for precise control of dissolved oxygen, pH, and sludge age, enabling operators to manipulate conditions to suppress filamentous bacteria. Other modifications include the use of anoxic zones (areas without oxygen) to promote denitrification (removal of nitrates) and further disrupt filamentous growth. In essence, modified activated sludge processes actively manage the microbial environment to prevent bulking sludge, whereas conventional activated sludge relies on a naturally balanced system that can be easily disrupted.