A ~8 MGD (~30,000 m³/day) activated sludge plant had been dealing with very slow sludge settling for many years. The issue affected day-to-day operation, but it also had a much larger implication: the future upgrade design of the facility.
When sludge does not settle well, plants often need to design around larger clarifier capacity, higher reactor volume, stronger RAS and WAS pumping, and more sludge-handling equipment. In an upgrade project, these assumptions can quickly become a multi-million-dollar decision
The plant operates as an oxidation ditch activated sludge process, where treatment conditions are created mainly through aeration patterns, mixing, oxygen gradients, and reactor operation. In this configuration, settling quality depends strongly on biomass structure, DO conditions, RAS rate, MLSS, and wasting strategy.
At the beginning of the monitoring period, the biomass showed poor settling characteristics: open, non-compact flocs, slow settling, and a high filament population dominated mainly by Thiothrix. This matched the operational picture. The plant was operating with a very high RAS flow.
30-min settling test - before process adjustment.
RAS flow was approximately 38,000 m³/day, compared with an average influent flow of about 19,500 m³/day - close to a 200% recycle ratio. For an oxidation ditch process, this was far above the expected operating range.
This high recycle ratio hydraulically overloaded the clarifiers and continuously returned large volumes of sludge back into the reactors. The result was weaker process conditions, lower effective retention, oxygen-limited zones, excessive filament growth, and poor floc structure.
Thiothrix filaments.
Maji helped connect the biological evidence with the process operation. The issue was not treated only as a clarifier-capacity problem. Based on the biomass and process picture, Maji suggested focusing on the RAS rate as a key operational lever, while continuing to monitor how the biomass responded.
The plant team reduced RAS flow from about 38,000 m³/day to about 17,000 m³/day, bringing the recycle ratio down to approximately 92% of the average influent flow at that stage.
The improvement was clear:
SVI improved from approximately 178 mL/g to approximately 78 mL/g.
Thiothrix dominance nearly disappeared
Flocs became more compact, with faster settling and a smaller sludge blanket
WAS flow / wasting flow decreased from about 2,400 m³/day to about 1,200 m³/day
30-min settling test - after process adjustment.
The wasting improvement was especially important. Because the sludge became more compact, the plant could remove the required solids with a much lower hydraulic volume. This can reduce the required capacity of WAS pumps, sludge-transfer systems, screw pumps, thickeners, and other sludge-handling equipment.
For a plant planning future expansion, this can affect the design assumptions behind the upgrade itself, including clarifier capacity, reactor volume, RAS and WAS pumping, MLSS targets, and sludge-treatment capacity. The potential impact can reach millions of dollars in avoided or deferred capital costs.
This was a team effort by the process engineer, plant manager, operators, and Maji. The plant team led the operational changes. Maji contributed the biomass-analysis layer: identifying what was happening inside the sludge, connecting it to the operating conditions, suggesting where the process should be examined, and helping follow the biological response after the changes were made.
The work is continuing, including further optimization of MLSS, RAS, and wasting strategy. The plant will also continue building its biomass baseline across different seasons and operating conditions.
The main lesson is clear: poor settling is not only a clarifier issue. It can affect the entire design logic of a plant upgrade. In this ~8 MGD (~30,000 m³/day) facility, connecting biomass analysis with process operation helped turn a long-term settling problem into a practical optimization pathway with major operational and capital-cost implications.