Anaerobic digesters are an energy-efficient process for waste treatment, especially in applications that convert biogas into useful energy such as electricity or heat. However, they require a homogeneously mixed process for maximum efficiency.
This function is typically provided in one of three ways: mechanical, gas or hydraulic mixing. All of these technologies claim to achieve similar results. They have varying capital and operational costs, the largest of which is typically energy. Of the three, hydraulic mixing offers advantages such as high mixing energy, effective mixing patterns, low maintenance, and easy installation. However, it tends to have high installed power consumption. But with the right control technology, hydraulic mixing can also address energy costs in a way that makes it a strong candidate for lowest life-cycle cost.
Hydraulic mixing recirculates reactor contents through a series of jet nozzles, typically driven by a chopper pump to prevent clogging from rags and other stringy solids in the sludge. Nozzle size, location, position and flow rate are engineered to produce velocities capable of inducing hydraulic flow up to ten times that of the pumped flow through the system. This combination of pumped and induced flow transfers sufficient energy to the reactor to keep contents mixed and solids suspended — the desired conditions for efficient anaerobic digestion. This process consumes significant energy, primarily to drive the chopper pumps. But the energy consumption can be offset by hydraulic mixing’s ability to operate intermittently, a feature not typically available with mechanical and gas technologies
Unlike other mixing technologies, the high mixing energy of hydraulic mixing can maintain an optimum mix in the digester without continuous operation, directly reducing the energy required to run the pump.
Implementing intermittent mixing depends on the ability to monitor the condition of digester contents, determine whether the mix is optimized, and use the data to control pump operation. This typically involves taking various samples within a digester to ensure that the sample temperatures and total suspended solids (TSS) values are consistent. If sample temperatures are within +/- 1 degree F and the TSS values are within +/- 10% of each other, it can be said that the reactor is completely mixed.
For automatic intermittent operation, the key performance parameter is temperature. To track it, Evoqua supports its Jetmix hydraulic mixing system with sensors at various locations within the digester that sample temperatures for comparison. Temperature data is analyzed by a local panel and used to control the chopper pump to maintain an optimum mix. The data can be fed directly to the plant’s SCADA system for monitoring as needed.
