Why Rectangular Clarifiers Are Underrated

Many engineers view rectangular primary and secondary clarifiers as relics of the past. My opinion is they are underrated. My opinion is based on performance data from plants throughout the United States and the results from hydraulic modeling. With ideal design or optimization, rectangular clarifiers can outperform their circular cousins in terms of effluent quality, surface overflow rates, capital costs, and long-term maintenance.

Rectangular Primary and Secondary Clarifiers

Rectangular primary and secondaries make the most of limited land area. Rectangular units are often built in smaller increments – so there is less impact when the largest unit is taken off line. For example, a plant with 8 rectangular clarifiers with the same surface area as 4 circular clarifiers retains 87.5% of its capacity available when a unit is out of service vs 75% with a circular. The difference is significant because circular clarifiers must be designed larger (at greater cost) to maintain engineering redundancy standards.

Circular clarifiers also require more complex splitter boxes and yard piping which can be a challenge to maintain. Rectangular units can be constructed with common inlet and outlet channels – further simplifying the design and future maintenance. In areas with unstable or corrosive soils, the yard piping may require pile supports, complex coating systems, and/or cathodic protection.

An area where rectangular clarifiers shine is the ability to install mid-tank baffling to improve settling. The idea is impractical with a circular clarifier because there is no place for support. In rectangular tanks, the opportunity to install a second or third baffle is straightforward. CFD modeling and field-testing show that rectangular clarifiers can achieve very high levels of performance across an even higher range of surface overflow rates, than clarifiers without mid-tank baffling

Before dismissing rectangular primaries or secondaries, the unique characteristics of the site and costs should be considered. In many cases, rectangular clarifiers can be made to provide elite solids capture, overflow rates, and primary or secondary effluent quality.