Aquagenics

A key clarifier design maxim states that solids removal efficiency is directly related to a clarifier's surface area available for settlement. A shallow, large surface area clarifier provides ideal conditions for better sedimentation performance, however, this requires a larger clarifier footprint and higher cost trade-off.

A cost-effective option for new plant and the best option for uprating existing plant is installing inclined tube settlers. Inclined tube settlers provide up to 6 times effective square metres of settling area per square metre of clarifier plan area. Where vertical upflow clarifiers operate without tube settlers fitted, settling particles form displacement upflow eddies which hinders the settlement of adjacent particles. This is compounded by settlement flow (gravity) being diametrically opposed to clarifier upflow, thus limiting a clarifier’s achievable upflow rate and solids separation performance.

Finely divided hydroxide floc and high proportions of colloids are usually the rate determining factors for solids removal. Large solids settle quickly and easily after entering a clarifier. Fine solids tend to remain in suspension taking much longer to settle, if at all. Tube settlers arrange clarifier upflow through inclined segregated honeycomb tubes. Significantly, settlement is with the flow within each tube, allowing higher upflow rates and bringing about contact clarification of finer flocs within the tube settlers. Settled particles combine to form agglomerates, the larger and more settleable of which slough against the upward flowing incoming water.

Tube settlers - before plant operation Tube settlers - after plant operation

A conventional water treatment plant clarifier’s flow rate capacity and performance - notwithstanding the available surface area - is also limited by less than full utilization of its plan settlement area due to unstable hydraulic flow profiles.

Clarifier area utilization and hydraulic stability are usually related. Unstable hydraulic conditions may be brought about by poor design but is more often affected by raw water (influent) diurnal temperature variations.

Apparently minor water density differences between a clarifier's contents and incoming raw water may lead to "boils", i.e. localised areas of high upflow rates which tend to invert a clarifier's suspended solids contents; this is usually characterized by low levels of "floc carry-over" during early morning, with worsening floc carry-over conditions throughout the remainder of the day.

Circular radial flow clarifiers are notoriously hydraulically unstable, with less than 50 per cent of the available clarifier area utilized in many instances (without tube settlers).

Tube settlers in operation

Tube settlers introduce a distributing resistance to flow which improves flow distribution and so improves clarifier area utilization. Improved clarifier inlet distribution uniformity stabilises the hydraulic flow profile and effectively dampens any localised hydraulic instability or "boils".

Improved clarifier inlet distribution uniformity also brings about a much denser and better defined sludge blanket yielding higher underflow sludge concentrations and lower (to one third) underflow waste volumes. This provides significant reductions in sludge management costs, water and wastewater volumes, pumping costs and brings about conditions where sludge blanket levels can be reliably detected hence facilitates effective automation of underflow wastage.

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