Regular followers know that the connection, the nexus, between wastewater and energy has been in our wheelhouse for several years. With more and more attention being paid to this by the industry and its suppliers, we felt it was time for a brief update.
This connection can generally be thought of as two-pronged. One involves technologies and processes to extract energy from the waste. This could be in the form of methane gas collected from the treatment process. Or it could be the combustible content of the sludge produced. Additionally, sludge has monetary value as a fertilizer.
The second path involves reduction of energy consumption in running the plant. Efficiencies are being sought in the equipment and in the processes being used.
Use of anaerobic digestive processes is frequently cited as one way to reduce energy costs while also generating increased amounts of burnable methane biogas. Stanford University engineers have developed a wastewater treatment process based on anaerobic bacteria. The process will be the foundation of a demonstration plant expected to be on line in fall of this year.
Traditional aerobic digestion requires large amounts of oxygen, provided by huge blowers requiring large amounts of electricity to operate. A key factor in the energy savings of anaerobic digestion is the elimination of the need for these blowers. Anaerobic processes also require less space for operation, as much as 40 percent less, saving money on plant construction and operation.
In a March article in Water & Wastewater International, several projects were cited where both prongs are being demonstrated to be effective. In one example, a Scottish treatment facility, the largest in the eastern part of the country, has achieved 85 percent energy independence through energy recovery and reuse. Anaerobic digestion is employed to increase biogas production. That gas is then used to fuel a high-efficiency CHP (combined heat and power) system that reduces the plant’s purchased energy requirements.
The Scottish plant has also reduced its costs for sludge disposal. Approximately half its volume of sludge is able to be used in agriculture.
The same article reports on a planned facility for Egypt that is being designed to achieve 100 percent energy self sufficiency. This will be achieved with a combination of physical-chemical, aerobic, and anaerobic biological treatment, and will optimize recovery of biogas and energy from the effluent. Designers project that the plant may even be able to sell energy back to the grid. A major impetus for this focus on energy self sufficiency is the irregularity of power availability from the local grid.
An area also often discussed with regard to energy savings in plants (not limited to wastewater treatment applications) is VFDs (variable frequency drives). VFDs have been gaining in use for the past decade. Plant Services provides a detailed mathematical description of how VFDs accomplish this energy savings. In simple terms, VFDs match the energy input to the motor’s load conditions, only consuming energy when it’s actually needed.
The energy saving capability of VFDs is addressed in detail in a Distributed Energy article. The article says that VFDs can cut energy consumption by 20 to 60 percent. Further, the article also indicates that the ability of VFDs to skip frequencies of resonance for pumps and motors can be a major factor in reducing maintenance costs.
In the W&WI article cited above, a Danish plant is put forth as an example of how variable speed drives have been in use for the past five years on all rotating equipment. Combined with other process improvements, the plant has been able to achieve production of 134 percent of its energy needs. The plant is considered an example of the achievement of twin goals, energy savings and increased energy production.
Those familiar with the activities taking place both in the US and overseas wastewater treatment industries agree that this two-pronged approach, the achievement of dual goals of energy savings and increased energy production will continue to grow in importance. The energy potential contained in the world’s wastewater stream is too large to ignore, and too valuable not to take advantage of.