Activated sludge process control via particle monitoring

Abstract

The interrelationships between activated sludge plant operation and floc characteristics were examined in order to determine process control mechanisms for the effective treatment of domestic wastewaters and surplus activated sludge. A quantitative indicator of the physiological floc characteristics was developed and termed the morphological index. This may be evaluated by plant operators using a simple assessment form and matching photographic guide of various floc structures. The morphological index and floc size distribution were found to be useful tools for activated sludge process control and monitoring the dewaterability of surplus activated sludge. It was found that floc size, morphology and in turn the dewaterability of surplus activated sludge varied with different operating conditions. Hydraulic retention times (HRTs) of < 10 hours and sludge ages of ≤8 days, resulted in the presence of small diffuse floc structures in the mixed liquor. The effluent quality was poor and the activated sludge had unfavourable filtering and dewatering properties. Switching plant operation to sludge ages in the region of ≥8 days and HRTs of ≥10 hours, was found to improve effluent quality and the dewaterability of surplus activated sludge. Here the flocs were large compact structures with short filaments protruding from the floc body.

Multiscale structures of biological flocs

“…For instance, a larger and more compact oc typically has better settleability and ÿlterability, reducing the required settler capacity and the need for occulants. Jiwani et al (1997) monitored several indices of oc morphology (size, compactness, and ÿlament numbers) and correlated them with the quality of the e uent and the dewaterability of the sludge. However, the e ects of oc structure, including the spatial distribution of microorganisms, intrinsic mass transfer resistance and possible substrate concentration distribution inside the oc, have not been incorporated into the activated sludge model (Bakti and Dick, 1992;Keller et al, 2002).…”

Effects of Salinity on Physical Characteristics of Activated Sludge Flocs

“…Regardless of the method employed, however, the ability of the sludge to dewater is highly dependent upon the physical structure of the individual sludge flocs in terms of their porosity and pemeability (Li and Ganczarcrlk, 1990;Ganczarczyk and Rizzi, 1996). Large, dense flocs with few protruding fïIaments are tolerant to shearing and compression during filtration dewatering procedures, whereas small, diffuse flocs with many filaments tend to shear during filtration leading to nIter medium and cake blinding (Jiwani, et al, 1997). Smailer particle sizes are also associated with poor dewatering (Hïggins and Novak, 1997~).…”

“…Although there exists many more aspects to the activated sludge process, such as nutrient removai, sludge handling, sludge incineration, process design, etc., only those aspects known to be directly affected by the physical structure of the activated sludge flocs were presented. This is because the floc characteristics are a reflection of the prevailing conditions in an activated sludge plant (Jiwani, et al, 1997). Now that the process has been described, and the relationship between the individual unit operations and the microbial aggregates identified, it is important to look at the structure and flocculation mechanisms involved in the formation of the activated sludge flocs.…”

The physico‐chemical characteristics of thermophilic aerobic sludges

“…These values need to be considered against the values obtained for activated sludge in previous studies. Snidaro et al 16 report a D‐50 value of 125 µm, whilst Jiwani et al 17 report modal values ranging from 150 to 600 µm, depending on the sludge age. This was confirmed by microscopic examinations of the sludges.…”