Biochar as a Consideration for Cost-Effective Small City Wastewater Treatment

Joseph Sabas, Iowa State University

In 2019 a team of Iowa State University engineering students including former ARTi interns Joseph Sabas and Denver Robak sought to help address the problems of wastewater treatment experienced by the small town of Rinard, Iowa. The tiny town of Rinard with a population of 50 citizens faces a confluence of challenges as do many such towns in the US and elsewhere. First of all, a projected continued population exodus does not bode well for future town revenues. Secondly, regulations regarding wastewater treatment have become more stringent and are predicted to continue to be so. Additionally, the existing system is over 30 years old and has not been actively maintained during this time. Correspondingly, an updated system would also have to be passive requiring little or no active maintenance as the town lacks both the people and the financial resources. Lastly, the improvements to the wastewater treatment system faced legal time deadlines and environmental stewardship concerns which were also important to the community and the student engineering team.

The team consisting of Sze Kei Lim, Danielle Ufheil, Breuklyn Opp, Denver Robak, Joseph Sabas, Kimberley Thong had a number of factors to consider simultaneously including remaining under budget, avoiding as many future maintenance costs as possible while providing the best option for Rinard’s environment. But, they also had a number of interesting options to choose from as well including biochar. Although, ultimately the decision went in favor of the installation of a UV disinfection system as it proved to have a decent balance of affordability, reliability and passive operation.

The team put considerable effort into examining the possibilities for biochar to act as a filter for the wastewater. Biochar has high absorption potential and can act as a soil amendment adding greater value plus its low cost. Or, as the Sabas Et al. team noted, biochar can be re-pyrolyzed, that is put through its oxygen-free, high-temperature baking production process an additional time to remove organic materials gained via the wastewater treatment process. However, the team in the end decided that more research was required in order to recommend biochar filtration with sufficient confidence into the town of Rinard.

That being said, we can still have a look at the promising work the team completed through their research into biochar’s potential for this type of application.

Constraints and Regulations

The Iowa Department of Natural Resources (DNR) maintains the NPDES, an effluent limit requirements permit. These requirements include limits of effluent levels of E. coli and pH and a number of other substances. In this case, the team tested for levels of ammonia, phosphorous and total suspended solids (TSS) all of which are regulated under the NPDES as well. These limits are:

• Ammonia 1.1 – 13.7 mg/L
• TSS <30 mg/L
• Effluent limitations for phosphorus equal to 1 mg/L

Essentially, ammonia levels can be from 1 to 14 milligrams per liter, total suspended solids (TSS) can be up to 30 milligrams per liter and phosphorus levels are allowed up to 1 milligram per liter. The engineering team planned to test if biochar and/or biochar-sand filter systems can reduce the effluent levels of these substances to amounts that meet the NPDES requirements.

In order to test biochar’s filtration potential, the team created three filtration columns, a sand filter (conventional filter), sand + biochar and a biochar only filter column.

The tests were timed and sought to see the effectiveness of the various columns on the filtration of wastewater resulting in effluents in terms of the amounts of ammonia, phosphorus and total suspended solids (TSS). Four pour-volumes (pourings), i.e. tests of added simulated wastewater fed through the columns were conducted as it was necessary to see any reductions in absorption rates over multiple use cycles.

In the case of ammonia, the biochar columns behaved well and showed a marked reduction. However, it was not enough of a reduction to meet regulation.

The first column (pour vol/rep) represents the number of pourings through the columns. The figures given are mg per Liter. The tables show that the pure biochar does reduce the concentration of ammonia particularly in the first pour volume at an average of 12.967 mg per Liter and 23.918 mg at the fourth and final pour volume. In the words of the engineering team “Overall, the biochar columns show the best adsorbance of ammonia, while the sand columns show the worst adsorbance of ammonia. DNR standards for ammonia allow for a maximum range of 1.1-13.7 mg/L, depending on the month. Both trials do not meet the ammonia DNR standards. Biochar media reduced the largest amount of ammonia, but was still far from meeting the required limits.” The biochar column does meet this in the first pour volume but loses its absorbency through repeated use. Biochar would need to retain its best results in this trial of 12.316 mg/L (or less) absorbency and not demonstrate such a rapid, drastic almost 100% decrease in absorbency for its viability for this application.

The biochar/sand column was very effective at reducing phosphorus including removing all of it in the first pouring although this absorption decreased over multiple pour volumes.

The pure biochar column did increase its absorbency for phosphorus over time but stabilized at an average of 12 mg/L. In terms of meeting regulations the team stated, “A general DNR standard for phosphorus in states where limits are imposed is 1 mg/L. Although the biochar media trials show significant potential in phosphorus reduction, none of the tested methods meet the DNR standard limits.” The sand/biochar blend shows the most promise in the case of phosphorus but still would need to maintain its best results from the first pour volume to be an option.

The total suspended solids (TSS) at first was high especially for the biochar columns yet decreased quite a bit over multiple pour volumes.

In the case of the suspended solids all media performed well and met standards with the engineering team stating,”Biochar TSS was especially high at first due to the amount of finer dust particles being flushed out. The stabilizing effect of the columns can be seen in the drastic drop in TSS in the effluent by the second pour volume. DNR standards for total suspended solids allow for a maximum effluent of 30 mg/L. All methods met the DNR standards by the second pour volume.” Overall, biochar on its own as well as with the sand mixture performed quite well in the case of TSS.

These results show that there is potential for biochar to be an effective, multi use and affordable material as part of a wastewater filtration system. Nevertheless, as stated previously the Iowa State engineering team decided that more research was needed before biochar could be recommended. It might be safe to assume that biochar as an option for cost effective wastewater treatment will however continue to be explored.