Rectangular vs. circular sedimentation basins

Both rectangular and circular configurations have been commonly used in sedimentation basins in both water and wastewater treatment plans. Historically, the selection of rectangular versus circular sedimentations has been based on past experience and the preference of the design engineer or design company. Initially, the water and wastewater treatment plant designs in the world were reasonably balanced between rectangular and circular when collector or scraper systems were designed from steel or bridge scraper for rectangular basins. With the introduction of biological treatment, corrosion became more of a concern and circular sedimentations became more popular. Non‐metallic chain scraper systems for rectangular sedimentations (like our DEWA system) were introduced more than 40 years ago and have overcome the corrosion problem, leading to a resurgence of rectangular sedimentations, particularly for large wastewater treatment plants. Let’s compare the benefits of each configuration.

Sedimentation basins operation and maintenance

Non‐metallic chain scraper systems having multiple flights (scrapers) installed at fixed spacing (typically 3 m) can accomplish both sludge and scum removal. The flights (scrapers) scrape the bottom of the tank and present consistent sludge flows to the hoppers (at the end of the basin). When the flights travel to the surface of the rectangular tank along the collector chain, the scum/foam is pulled by the flights to the scum pipe. Removal of scum and floating solids is almost always easier with rectangular clarifiers, which are known to provide more effective scum/foam trapping via rotating scum pipe. In contrast, the circular scraper systems have only two rotating collector arms with multiple scrapers on each arm that plough the settled sludge to a hopper at the centre of the tank. A single cantilevered skimmer arm rotates at the surface to collect scum and concentrate it in a hinged skimmer, from where it is then dragged up a scum beach and trough.

Non‐metallic chain scraper systems for rectangular clarifiers remove all sludge across the collector at a constant speed of approximately 1,2 m/min. Circular collectors move sludge at a much faster rate at the perimeter of the collector than at the centre of the collector. It is generally accepted that scraper speeds greater than 1,8 m/min can cause re‐suspension of settled solids and that will affect negatively the efficiency of the sedimentation performance.

Where large collectors are to be built, scum control and the effectiveness of wind‐blown mechanism might be difficult with circular clarifiers especially when there is no wind or in covered tanks.

Sedimentation performance

The shape of the sedimentation basins alone does not exhibit observable differences in clarifier performance as long as the sedimentation basins are well-designed.

A widely documented clarifier investigation conducted by the Sanitation District of Los Angeles County (LACSD) has shown that shallow rectangular Sedimentations (3m deep) with concurrent sludge removal perform effectively at surface overflow rate (SOR) as high as 7500 m³ per day / m². In fact, compared to data obtained from a series of previous circular clarifier studies, the effluent suspended solids (ESS) concentration from the 3 m rectangular clarifier is equally effective to circular Sedimentations of 5.5 m depth, which suggests that the rectangular Sedimentations can be built shallower to achieve the same effluent quality and subsequently result in cost savings.

According to that in order to handle 4300 m³ of sludge with circular sedimentation you need to have 30 m dim x 5.5 m high = approximately 600 m³ of concrete + construction cost. By using rectangular sedimentation, you need to have one 8x80x3 m basin to handle 4300 m³ of sludge = 260 m³ of concrete with a simple construction design. That means half of the cost will be cut according to that.

Clarifier hydraulics

The shape of the rectangular sedimentations provides a longer path for the wastewater flow and the suspended solids to travel, and subsequently longer detention time which warrants less short-circuiting and more sludge settling compared to the centre-feed/peripheral overflow circular sedimentations. In addition, flow distribution among several sedimentations is usually more even and often requires less head loss for rectangular sedimentations.

Construction

Rectangular sedimentations typically require less land than circular sedimentations for a similar surface area (22% less). The reduction becomes even 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. Construction cost is also reduced as a result of the common concrete walls.

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

Many sedimentations now need to be covered for odour or volatile organic compound (VOC) control. Covers for rectangular sedimentations (chain scraper systems) are much easier to design and install as well as being less expensive.

Installation and maintenance cost

Installation of a non-metallic chain scraper system is very fast and cheap with the DEWA chain scraper system you can install a standard sedimentation basin 8x40x3 m 4-shafts system within 12-16 hours with a team of 4 workers and without any heavy-duty cranes. On other hand with circular sedimentations, you need at least one week to install one 30 m diameter basin with heavy-duty cranes and a team of 4 workers including a special welder.

Same things during the maintenance time you need to empty the basin and lift the heavy parts of the basin with a circular basin but with the DEWA chain scraper system most of the time you don’t even need to empty the basin for maintenance, because you can change the scraper flight and do all the needed cleaning in the surface of the water.
According to the calculations, the cost of installing the chain scraper system is about 80% cheaper than the cost of installing the circular scraper.

Lifecycle cost

The life cycle cost of rectangular ponds is much cheaper as compared to circular basins. Rectangular configuration can be higher in capital cost (filter equipment, civil cost and pumping station cost if required) and total life cycle cost. with DEWA non-metallic chain scraper system, the spare part prices are very low because they are mostly light parts easy to replace, as we have full control of our system the possibility of high damages is very low. In addition to that with the non-metallic system, the energy consumption is much lower if we calculate the cost for a long life period (20 years) it will be 60% less than the circular scraper.