Question

Circular Clarifiers and Rectangular clarifiers are facilities are used for clarification of water and wastewater during treatment.

1- Write short notes on each one of them with more emphasis on the similarities and differences in treatment processes.

2- Advantages and disadvantages between the circular clarifiers and Rectangular clarifiers..

Answer 1

2. The advantages and disadvantages between circular and rectangular clarifiers are-

Item	Rectangular Clarifiers	Circular Clarifiers
Advantages	Less land is required for construction of multiple units. Potential construction cost savings due to the use of common walls between individual tanks. Longer flow path minimizes short-circuiting. Higher effluent weir loading rates is acceptable in this case. It has better sludge thickening.	The shorter detention time for settling sludge favors to use as secondary clarifiers. It is more simple sludge collection system. Easier to accommodate in-tank flocculation chamber – a advantage for activated sludge settling. Requires lower maintenance. Easier to remove heavy sludge in this case.
Disadvantages	Longer detention time of the settled sludge – not favorable for plants with septic wastewater influent. Less effective for high solids loading conditions.	It has higher short-circuiting potential. It has higher flow distribution head losses. The small circular tanks require more yard piping than rectangular tanks of similar size.

1.The similarities and differences in treatment processes of circular and rectangular clarifiers are-

The both rectangular and circular configurations have been commonly used in clarifier basins.

From past, the selection of rectangular versus circular clarifiers has been based on past experience and the preference of the design engineer or design company.

With the introduction of biological treatment, corrosion became more of a concern and circular clarifiers became more popular in recent years.

The similarities and differences are-

1.Clarifier Performance
Shape of the clarifier alone does not exhibit observable difference in clarifier performance as long as the clarifiers are well designed.

Study reveals that the rectangular clarifiers up to 3m deep with concurrent sludge removal perform effectively at surface overflow rate (SOR) as high as 2000 gpd/ft2.

The data obtained from a series of circular clarifier studies it was observed that the effluent suspended solids (ESS) concentration from the 3m rectangular clarifier is equally effective to circular clarifiers of 5.5m depth, which suggests that the rectangular clarifiers can be built shallower to achieve the same effluent quality and subsequently result in cost savings.

2.Clarifier Hydraulics

The shape of the rectangular clarifiers provides a longer path for the wastewater flow and the suspended solids to travel and has longer detention time and more sludge settling compared to the circular clarifiers.

In addition the flow distribution among several clarifiers is usually more even and often requires less head loss for rectangular clarifiers.

3.Construction
Rectangular clarifiers requires less land than circular clarifiers for a similar surface area.

The reduction becomes more significant in a multiple-unit design where common concrete walls are used between rectangular basins.

The resulting land availability is a major advantage for treatment plant layout and the construction cost is also reduced as a result of the common concrete walls.

The even flow distribution configuration for rectangular clarifiers requires simple and less expensive pipe work layout and pumping requirement as compared to circular clarifiers where the pipes requires more complicated layout pattern and a separate pumping station as well.

4.Treatment, operation and Maintenance
The removal of scum and floating solids is always easier with rectangular clarifiers which are known to provide more effective scum or foam trapping via rotating scum troughs.

But in case of the circular scraper systems have only two rotating collector arms with multiple scrapers on each arm that plow the settled sludge to a hopper at the center of the tank.

A single cantilevered skimmer arm rotates at the surface to collect scum and collected it in a hinged skimmer from where it is then dragged up a scum beach and trough.

The non‐metallic chain and flight systems for rectangular clarifiers remove all sludge across the collector at constant speed .

The circular collectors move sludge at much faster rate at the perimeter of the collector than at the center of the collector.

It is accepted that scraper speeds greater than 6 ft (1800 mm)/min can cause re‐suspension of the settled solids.

The most negative aspect of rectangular clarifiers is the relatively frequent need to replace collector chains.

5.Lifecycle Cost

The lifecycle cost for both the rectangular and circular configurations are generally in similar range.

The rectangular configuration has lower capital cost and total lifecycle cost.

The circular clarifier can be lower in O&M cost such as mechanism replacement and maintenance costs.