ARTICLE
Geotube® vs. belt press. Which one wins for sludge dewatering?
Gravity or pressure—which is more efficient when dewatering wastewater sludge? Both are proven methods to remove water from slurry and produce a cake that can be handled for disposal or land application. But there are several important economic and operational factors to consider before choosing between a Bishop Solids Management Solution (BSMS) that uses semi-passive, gravity-based Geotube® sludge dewatering, or a pressure-based mechanical system such as a belt press.
Sludge Dewatering Cost
There are only four major components to the BSMS — a non-mechanical polymer mixing and activation system, feed pumps, hoses and Geotube® containers. This low-complexity system not only keeps capital costs low, it also requires little electricity for pumps and control equipment. By comparison, a belt press is a complex system that is costly to rent or buy and requires a significant amount of electricity to operate.
Easy to operate
Mechanical belt press systems typically require full-time staff to monitor and adjust the process, especially if the sludge varies in organic matter concentration and percent solids. The BSMS requires continuous staffing during setup to install the equipment, optimize sludge flow and polymer feed rates, but after that, the system can function largely automatically. Sludge can be pumped continuously and systems can be set up to automatically adjust polymer dose and feed rates according to sludge characteristics. Technicians may only need to monitor the system every few hours.
Sludge Dewatering performance
When dewatering biosolids, both the BSMS and belt presses can achieve about 18-20 percent solids – depending on the characteristics of the sludge and polymer that is used. However, if time and space permit, Geotube® containers can remain at the site to continue dewatering by gravity and potentially achieve 25 percent solids or more. This simple step to further reduce residual mass can help dramatically reduce hauling and disposal costs of the dewatered solids.
Retention of solids and contaminants
The BSMS with Geotube provides wastewater sludge dewatering and containment in a single step. Optimized polymer conditioning helps to retain up to 95% of total suspended solids as well as nutrients, metals and other contaminants. This containment also minimizes or eliminates the potential for spills and operator exposure to odors, pathogens and contaminants. Solids from a belt press, on the other hand, are open to the atmosphere and can potentially expose operators to spills, odors and contact with hazardous material. Additional handling is also needed to move solids from the belt press to a storage or staging area for disposal.
Processing rates
The BSMS is capable of achieving continuous flow rates of up to 2,000 gpm (7,570 L/min), enabling large volumes of sludge to be pumped and dewatered very quickly. Portable truck-mounted belt presses have a maximum flow rate of about 150 gpm (568 L/min)* and require significantly more time and cost to complete a dewatering project.
*Based on information from: Use of Geotube® dewatering containers in environmental dredging, B.J. Mastin and G.E. Lebster
