As companies increasingly look to capitalize on incentives to bring renewable fuels to market, wastewater treatment considerations are emerging as key elements in developing designs for these processes. Several challenges must be addressed in removing impurities from these wastewater streams.
A treatability test for one such wastewater source revealed some unique challenges for a novel renewable fuel facility utilizing blends of corn-based feedstock. The proposed wastewater stream was found to have approximately 10,000 milligrams per liter (mg/L) BOD (biological oxygen demand) and 15,000 mg/L COD (chemical oxygen demand). BOD and COD are measurements of wastewater quality that must be treated and removed before the water can be released as effluent or recirculated for reuse inside the plant. BOD is a measurement of the amount of oxygen required by microorganisms to break down organic material while COD represents oxygen required to chemically oxidize organic material.
With both BOD and COD at high levels, a common treatment approach is to utilize anaerobic processes followed by aerobic polishing. Anaerobic treatment is used as the first step because it breaks down organic material efficiently and does not generate as much biomass or require dissolved oxygen addition via blowers or mechanical aerators. This results in a second, more economical, aerated polishing step — often a conventional activated sludge system — to further remove impurities to meet discharge limits.
However, the treatability testing found that inhibitory compounds, including a wide range of phenols, impaired the anaerobic microorganisms. In addition, novel processes are susceptible to operational upsets or changes in water quality, which create conditions that anaerobic microorganisms are more sensitive to. For example, temperature variations and high pH levels can upset sensitive anaerobic treatment processes, causing inhibition and less efficient treatment.
Upon this conclusion, the design pivoted to an aeration-only treatment option. However, this approach called for high energy demand for the blowers needed for oxygen introduction. This approach was further complicated because of the high volumes of sludge that would be generated. The sludge would need to be dewatered and then disposed of off-site.
In addition, the loss of an anaerobic treatment phase eliminated the potential of creating renewable natural gas from the biological degradation of organic material in the wastewater. That beneficial energy source could have been utilized for heating or to power on-site utilities within the plant.
The overall conclusion following the initial design is that aerobic treatment as the sole biological process provides a reliable and low-risk design that can meet discharge limits. The tradeoff is that it does impose a higher energy use and produces large quantities of solids that require disposal without the benefit of renewable biogas production.
Challenge or Opportunity?
When dealing with renewable streams, it is important to get treatability testing completed early to inform the design. Typical parameters used for fossil fuel refinery wastewater are not uniformly applicable to renewable wastewater because the feedstocks and upstream processes generate waste streams with different characteristics. Treatability studies are vital to help establish the design criteria and kinetic coefficients that may result from these new wastewater streams.
As demonstrated in this project, testing and design is a truly iterative process that involves detailed analysis of the data that informs the relative effectiveness of various treatment pathways. Though this treatability study indicated that the initial two-step anaerobic-to-aerobic process wasn’t feasible for this specific application, it has resulted in further reexamination of options that could bring both treatment options back into consideration in future efforts. For example, changes to upstream processes could be evaluated to reduce concentrations of known inhibitory compounds being sent to wastewater. Methods to allow for consideration of anaerobic treatment could have positive impacts from both a capital and operational expense perspective.
Though anaerobic or aerobic treatment are the primary options in wastewater treatment, there are many variations and combinations of those pathways that can provide the desired results that are needed. Possible pathways to be explored include incorporating media in an integrated fixed-film activated sludge system or incorporating powder- or granular-activated carbon into the wastewater design to further enhance treatment and reduce toxicity issues. A membrane bioreactor could also be considered if the treatment requirements target a higher quality effluent or applications for wastewater reuse.
The next phase of the project will involve definition and refinement of treatment options based on data obtained from further treatability studies. Though it is likely that an aeration-based conventional activated sludge system will ultimately be the most effective treatment option for this particular wastewater stream, the additional results from follow-up treatability testing will provide the data needed to make the most informed business judgment on the best path forward.
Renewable fuel is gaining momentum as an attractive option for companies seeking to meet growing market demand for lower carbon intensity fuels.