Every industry’s onsite wastewater treatment has its unique conditions and challenges. For distilleries, high levels of biochemical oxygen demand (BOD), nitrogen, total dissolved solids (TDS), and total suspended solids (TSS) put extra pressure on optimal biological treatment requirements. Here are some techniques that can help distillers put a little extra money in the bank while meeting the challenges of environmental discharge requirements.
One Process’s Trash Is Another’s Treasure
Once a distilling batch process is complete, its byproducts need to be treated for environmentally acceptable disposal or reuse. Even after leftover solids (grains) from the distilling process are removed and dried by filter presses to be used for other purposes — e.g., animal feeds or fertilizers — the remaining wastewater can still contain high BOD, nitrogen, TDS, and TSS concentrations. That BOD holds great potential for generating biogas that can be used to fuel boilers for process steam or heating or to support the facility’s own electrical generation capabilities. The nitrogen is removed downstream by anoxic-oxic biological treatment.
The Biology Of Biogas Sustainability And Water Reuse
Treatment of wastewater streams from the process and from post-production wash-down procedures depend heavily on the treatment or repurposing approach chosen. For biogas applications, some distilleries are using anaerobic microorganisms in a sealed environment to break down biological components into carbon dioxide plus methane used to fuel onsite boilers. Anaerobic contact processes are often chosen for this step and they can use jet mixers for cost-effective hydraulic mixing to maintain consistent distribution that helps anaerobic microbes be most effective (Figure 1).
The design of the jet mixer system should include the ability to throttle back the hydraulic mixing once the process is optimized and let the biogas generation contribute to overall process mixing, thereby further conserving energy costs. Beyond the anaerobic process there is the need to address the high concentration of nitrogen compounds that have been converted to ammonia during anaerobic degradation. Often, two-stage anoxic-oxic processes are used to aerobically convert ammonia to nitrate and then use the nitrate in the anoxic reactor to complete the nitrogen removal process. Slot- injector aerators/mixers have been used successfully to efficiently provide oxygen that helps aerobic bacteria finish cleaning the remaining wastewater stream as well as provide proper mixing in the anoxic step. Some distilleries use the anoxic-oxic process while also incorporating a membrane bioreactor (MBR) design where solids separation is accomplished via ultrafiltration or microfiltration membranes. These membranes effectively clean the water to the point of making it acceptable for non-critical wash-down water or for graywater uses. For MBR processes, slot injectors, which do not experience depressed alpha factors, can help to maintain high O2 transfer rates in the highly concentrated biomass environments and keep membranes clean by scouring them with cross-flow and air bubbles (Figure 2A). In cases where high TSS could accelerate clogging of fine- bubble diffusers in as little as a year, using jet aerators or slot injectors with built-in back-flushing capabilities can help keep the jets open for optimum aeration and hydraulic mixing capacity (Figure 2B). Whether a process involves mixing equalizing tanks, anaerobic treatment, anoxic tanks, or anaerobic reactors, proper jet nozzle specification and sizing should always be based on mixing intensity ranges and other empirical application parameters.
The Physics Of Biogas Sustainability
Being most efficient in wastewater biogas applications depends on matching the physics of the mixing process to the biological demands of the process. Here are four important areas to take into consideration when looking to optimize biogas generation from anaerobic reactors:
Power. Mixing intensity/power density requirements— expressed as brake- horsepower-per-million-gallons or watts-per-cubic-meter — will vary based on tank volume, hydraulic retention time, and specific wastewater characteristics. Work closely with an experienced supplier to calculate the appropriate need for the application.
Distribution Energy. Once the mixing intensity/power density is calculated, harnessing maximum effectiveness from it depends on having even distribution throughout the basin, not just concentrated in some areas atthe expense of others. Jet mixing technology can distribute the energy from a single pump toas many as 100 outlet locations throughout the cross-section of a tank to optimize blending.
Turnover Time. One measure of mixing thoroughness is how many times the volume of the tank is turned over during its hydraulic retention time (HRT), expressed as the volume of the tank divided by total circulation flow rate provided by the jet mixer. For jet-mixed anaerobic applications, a good target is to circulate between three times and five times the volume of the tank over the course of one HRT duration.
Velocity. The thrust of the wastewater being pushed out of the mixing jets, combined with the pattern of the jet nozzles, impacts the mixing characteristics of an anaerobic reactor. Computational fluid dynamics (CFD) modeling is a good way to project, and later confirm, the needed fluid velocity based on the composition of the wastewater and the need to keep light solids in suspension (Figure 3).
The Economics Of Biogas
Sustainability. Whether jet nozzles are used exclusively for extracting biogas from wastewater streams in anaerobic reactors or reducing BOD in aerobic treatment basins, the following features enhance their viability across a range of industrial wastewater applications:
Construction Materials. Corrosion-resistant and abrasion- resistant materials used for pipe and support infrastructure and nozzle construction assure a long low-maintenance installation life — on the order of 20 years or longer.
Pump Power Requirements. By being able to power an entire treatment basin with just one pump/motor as opposed to other mixing approaches, jet mixers can reduce both capital expense (CAPEX) and operating expense (OPEX) as compared to other options for anaerobic contact processes in distillery wastewater applications. The use of variable frequency drives on the pumps for turndown once gas production is optimized enhances energy savings further.
Installation Efficiency. Jet mixing and aeration systems for aerobic treatment that can be installed as skid-mounted infrastructure can speed and simplify the process of upgrading existing basins. The ability to mount pumps and blower connections outside the basin also simplifies both initial installation and future maintenance.
Process Efficiency. Slot injectors, which include the option of turning down both blower and pump mixing power in response to changing oxygen demand, can deliver additional OPEX benefits — whether the process includes hydraulic mixing with or without aeration.
