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The focus of wastewater treatment over the last few decades has been to efficiently collect and treat water prior to discharging it to the environment. These processes invariably produce an effluent sludge material in which all of the pollutants, pathogens and other substances are not degraded completely during the treatment process. Historically, little attention has been paid to the disposal of this residual material. This is now changing, however, for various reasons.
The beneficial reuse of this unavoidable byproduct increasingly is seen as an economically and ecologically important part of the wastewater treatment process. Additionally, sewage sludge is wet and therefore very expensive to transport; it does not compact very well; it may contain a considerable amount of pathogens, organic pollutants and heavy metals; and it has the potential to give off an undesirable odor.
Wastewater treatment plants (WWTPs) have been able to land apply much of their sewage sludge on local farms or dump it at landfill sites. These practices, though, are coming under increasing amounts of public fire and in some cases are becoming banned altogether. Thus, additional treatment steps are required to convert sewage sludge into valuable biosolids that can save money and be reused beneficially.
To accomplish this, the moisture content, volume of pathogens and overall odor must be reduced. The calorific value and biological stability of the effluent solids must be increased. When these treatment conditions are met, the end product is a dried biosolid that can be reused as a fuel in waste-to-energy plants, coal-fired power plants or cement kilns, and also can be used as a Class A fertilizer for agricultural use or land application. Finally, the volume of the wet sludge is reduced dramatically during its conversion to dried biosolids, which dramatically reduces handling and hauling costs.
High operating costs, energy consumption, emissions and the carbon dioxide (CO2) footprint to convert the wet sludge into dried biosolids have hindered a broad use of traditional sludge treatment processes such as thermal dying, composting and lime stabilization. In this context, an energy-efficient sludge drying technology is available that has been popular for many years in Europe and has been growing in popularity in the U.S.: the Thermo-System active solar sludge dryer. Since its introduction in Europe in the early 1990s, the technology has established an international installation base of more than 150 installations; there are an additional 17 Thermo-System solar dryer projects in operation or under construction in the U.S.
These solar dryers are located in a range of areas—from the cold and wet climate in the Northeast to the dry and hot climate in the Southwest. More than 90% of the energy used in drying the sludge is provided by the sun free of charge and with no CO2 production. As a result, the total dryer consumption is 30 to 40 kWh of electrical energy per ton of water evaporated from the sludge.
The Thermo-System process fundamentally consists of a greenhouse-type structure with a concrete floor surface and containment walls that is referred to as a “drying chamber.” There is a small robotic vehicle, the Electric Mole, which aerates and mixes the sludge contained inside the drying chamber. The drying chamber includes air vents on one gable and exhaust air fans at the opposite gable for air exchange between the drying chamber and the ambient environment. Fans mounted on the trusses of the drying chamber provide turbulent air movement over the sludge to break up any boundary moisture blankets that could form on top of the sludge, as well as evaporate water through forced convection. Climatic sensors located inside and outside the drying chamber relay relevant process parameters to a programmable logic controller (PLC). The PLC uses a propietary and sophisticated software program to monitor and automatically control all aspects of the drying process to fully optimize drying time (see Figure 1).
To operate the system, liquid or mechanically dewatered sludge is spread on the floor of the chamber. This is typically done manually with a front end loader or dump truck, but it also can be done automatically using pumps. Once the chamber has been filled with sludge, the operator enters the dry solids concentration of the incoming sludge on the PLC. From there, the PLC automatically controls all aspects of drying until the desired dry solids concentration is reached. At that time, an alarm alerts the operator that the chamber is ready to be emptied and filled with new, wet sludge. The automatic drying operation coupled with a low number of moving parts results in low requirements for operator attention and system maintenance.
Test results from two solar sludge drying trials conducted at an 8,500-sq-ft, full-scale, solar dryer installation in California from April 2009 to June 2009 proved that the technology is capable of producing a dry, safe and high-quality end product similar to conventional gas-fired thermal dryers.
In Trial No. 1, 210 cu yd of sludge was loaded into the dryer at 17.8% dry solids and reached 75% dry solids in 18 days. In Trial No. 2, 210 cu yd of sludge was loaded in to the dryer at 14.7% dry solids and reached 75% dry solids in 14 days. In both trials, pathogen levels were reduced to those required by the U.S. Environmental Protection Agency (EPA) for Class A biosolids.
It is widely expected that the high costs associated with disposal of sewage sludge containing high moisture content will continue to increase. Trucking companies will be forced to charge more as fuel prices rise, and as landfills fill they will charge higher tipping fees or stop taking wet sludge altogether. At the same time, it is expected that the EPA will put pressure on new and existing WWTPs to convert their sludge into biosolids for beneficial reuse. This will prompt a large number of WWTPs to search for technologies capable of consistently drying sludge to low moisture levels and reducing pathogen content.
The Thermo-System’s performance coupled with its low energy, operator attention and maintenance requirements are expected to render it a popular technology with treatment plants across the U.S. looking to meet their sludge treatment needs in an economical and environmentally friendly manner.