There are several advantages to each technology so it’s important to research all the available options
The benefits of thickening biosolids are well-known in the municipal wastewater community. The simple process of taking sludge from 1 percent to 5 percent solids results in a massive 80 percent reduction of the fluids in the sludge. The positive results of the volume reduction quickly ripple through the value chain, driving operational savings and positive environmental benefits. For solids being taken off-site, truck traffic is reduced and disposal costs are often slashed. For sludge that needs further processing in digesters, lower volumes can allow for improved retention times and help avoid investments in additional digester capacity.
A wide range of options is available to engineers and plant operations managers when choosing their sludge thickening strategies. These can be as simple as static gravity settling, to more advanced technologies like gravity belt thickeners, rotary drum thickeners and centrifuges. Each technology has some advantages and considerations that must be weighed when defining a biosolids strategy. We will look at some of the more advanced options.
Gravity belt thickeners have been a standard in the industry for many years and are a well-known technology. They are equipped to handle high sludge loading rates as thin as 0.5 percent and process it in a continuous manner. However, they have some inherent drawbacks in their method of operation. First, a gravity belt thickener is an open system, which can lead to high odors as well as a generally wet operating environment. This means installations must be indoors with adequate odor control accommodations, as well concrete work for fluid containment. These systems also tend to have high water requirements for continuous flushing of the belt. The overall footprint of the gravity belt thickener, as well as the site work required for a new installation, tends to drive higher capital construction costs compared to other options.
Rotary drum thickeners, by comparison, are closed systems where the entire sludge thickening process is contained within the equipment. They are also designed for continuous operation with an input sludge consistency as low as 0.5 percent sludge with the ability to thicken from 5 to 15 percent sludge depending on the type. Rotary drum thickeners require less floor space while maintaining the same processing capability as other technologies, and tend to have a lower overall capital cost. Additionally, well-designed rotary drum thickeners, like the IFT thickener from JWC Environmental, can use much less polymer due to employing woven wire mesh as the screening media. These benefits drive down both the initial installation costs, along with the ongoing operational requirements.
Centrifuges are a proven effective technology for thickening sludge. They can produce much thicker sludge than gravity belt thickeners or rotary drum thickeners, but this benefit comes with additional costs. Centrifuges require significantly more power since they operate at high speeds to dewater the biosolids. They also tend to have a much higher initial purchase price due to the precise nature of construction — and for proper operation of high-speed rotary equipment. Care must also be taken when using a centrifuge to ensure solids and large rags are not introduced into the equipment, as these can quickly clog and damage a sensitive centrifuge causing expensive repairs. Often an in-line grinder, such as the Muffin Monster from JWC Environmental, is used to protect centrifuges and break down any large solids.
The sizing of centrifuges is highly dependent on the input conditions and output requirements of the sludge. One strategy employed by some facilities is to pre-thicken sludge with a rotary drum thickener to get it up to the 5 to 7 percent range prior to sending it on to a centrifuge. This simple investment can drastically slash the initial capital costs for centrifuges. It will also decrease the ongoing energy costs associated with operating a smaller centrifuge.
While thickening biosolids is absolutely imperative for efficient operation of a resource recovery facility, there are also multiple factors that can lead to unparalleled success or costly ongoing problems. The experts at JWC Environmental (www.jwce.com) are a great resource for discussing your requirements and helping you discover what works best for your unique situation.