2020 Excellence in Environmental Engineering and Science™ Awards Competition Winner

Grand Prize - University Research

Hybrid Adsorption Biological Treatment System (HABiTS) for Nitrogen Removal in Onsite Wastewater Treatment

Entrant: University of South Florida
Engineer in Charge: Sarina J. Ergas, Ph.D., P.E., BCEE
Location: Tampa, Florida
Media Contact: Sarina J. Ergas

Entrant Profile

Project Description

Introduction

Onsite wastewater treatment systems (OWTS; aka septic systems) treat approximately 25% of US domestic wastewater and are significant sources of nutrient and fecal indicator bacteria contamination of surface and groundwater. Advanced onsite treatment technologies have been developed; however, these systems often fail because they are difficult for homeowners to operate and maintain. "Passive" biofilters have been developed that are similar in operations and maintenance (O&M) to septic systems and can be retrofitted into septic system infrastructure (Figure 1). Passively aerated nitrifying biofilters are followed by submerged denitrifying biofilters with wood chip media, which slowly releases organic carbon for denitrification. This avoids the need for forced aeration and complex chemical feed systems (e.g., for methanol).

However, OWTS are challenged by diurnal loading rates and long idle periods (e.g., during vacations). This results in overloaded microbial processes during high loading rate periods and die-off of beneficial microbes during idle periods.

Innovation

This project developed Hybrid Adsorption Biological Treatment Systems (HABiTS) for OWTS. A conceptual model of HABiTS is shown in Figure 2. Low cost ion exchange (IX) materials, clinoptilolite and tire chips (Figure 3), are included as biofilm carriers. When loading rates are high, excess nutrients partition to the IX media. When loading rates are low, nutrients desorb from the IX and are degraded by the biofilm. This results in a steady source of substrate to the microbial population. No fresh IX material is added and no brine waste is produced since the IX medium is "bioregenerated" by the microbes. Innovative aspects of the project include:

• Nitrifying biofilters incorporated low cost natural zeolite, clinoptilolite, with a high IX capacity for ammonium (NH₄⁺) to adsorb NH₄⁺ during high loading periods and desorb NH₄⁺ during low loads.
• Denitrifying biofilters incorporated scrap tire chips, which have a moderate IX capacity for nitrate (NO₃^-). Scrap tires also release biodegradable dissolved organic carbon to partially support denitrification.
• Denitrifying biofilters also incorporated elemental sulfur (S⁰), a petroleum refining by- product, for sulfur-oxidizing denitrification (SOD). SOD has lower sludge production than heterotrophic denitrification, reducing O&M (e.g., for backwashing) and carry-over of organic carbon to the effluent.
• Crushed oyster shells were added as a slow release alkalinity source, to augment alkalinity consumption by nitrification and SOD.
• Effluent disinfection for onsite water reuse was done using simple chlorine tablet feeders.

Results

Proof-of-concept studies tested a tire-sulfur heterotrophic-autotrophic denitrification (T-SHAD) biofilters with scrap tire, S⁰ and oyster shell media. Adsorption isotherms showed that scrap tire chips have an adsorption capacity of 0.66 mg NO₃^--N g^-1. Microcosm experiments showed that scrap tires leach bioavailable organic carbon that supports mixotrophic metabolism, resulting in lower effluent SO₄^2- concentrations than SOD. T-SHAD columns had low effluent NO₃^- concentrations even under highly variable NO₃^- loads (Figure 4).

In a follow up study, transient loads of domestic wastewater were applied to bench-scale nitrification-denitrification biofilters with and without IX media. Incorporation of clinoptilolite resulted in steady low NH₄⁺ concentrations compared to the conventional column, even during the start-up period when microbial biofilms were being established (Figure 5). Over a 15 month study, total nitrogen (TN) removal in HABiTS was significantly enhanced compared with the conventional process, particularly under high NH₄⁺ loads.

Funding from a USEPA Nutrient Center grant allowed us to carry out pilot-scale HABiTS studies at Hillsborough County's Northwest Regional Water Reclamation Facility in Tampa Florida (Figures 6). Two pilot HABiTS, with and without recirculation (Figure 7), were tested over 1.5 years. Recirculation significantly improved NH₄⁺ removal (~85%) compared with HABiTS without recirculation (~50%; Figure 8) by reducing the organic load to the nitrifying biofilter, reduced clogging and enhanced fecal indicator bacteria removal. Consistently low effluent NO₃^- and NO₂^- concentrations were observed throughout the study. The results show that HABiTS is a low-cost, low-energy and robust process that can consistently achieve advanced secondary standards under transient loading conditions. Our team is currently carrying out economic and life cycle assessment studies of HABiTS for onsite water reuse.

Benefits

OWTS provide sanitation in economically challenged rural and suburban areas and developing countries that lack access to centralized wastewater treatment facilities, financial resources or a skilled workforce. However, they are diffuse pollution sources that contaminate drinking water sources and promote eutrophication, sea grass mortality and harmful algal blooms. Application of HABiTS for OWTS has the following benefits:

• Reduced soil, surface water and groundwater contamination from septic systems.
• No complex chemical feed systems or mechanical aeration with associated high energy costs.
• The product of biological nitrogen removal, N₂, is a benign; SOD has low N₂O emissions.
• Biofilter media consists of low cost natural zeolite (an additive in kitty litter) and waste materials (S⁰, tires, shells).
• IX media is "bioregenerated" by microbial biofilms; no fresh media is added and no waste brine is produced.

The project trained 6 undergraduate and 7 graduate environmental engineering students to develop sustainable OWTS solutions (Figure 9). Three MS theses, two PhD dissertations and three peer-reviewed articles were derived from this research. Students gave presentations at regional, national and international meetings of WEF, IWA, AIChE, NEHA and SSSA. The pilot-plant served as a demonstration site, which hosted researchers, regulatory and utility staff and consulting engineers. Our team also organized a successful workshop on OWTS at the WEF Nutrient Symposium in Ft. Lauderdale Florida in 2017. Dr. Ergas guest edited a special collection of the ASCE Journal of Sustainable Water in the Built Environment on Onsite and Decentralized Wastewater Management.

Figure References

1. Aponte-Morales, V.E., Payne, K.A., Cunningham, J.A, Ergas, S.J. (2018) Bioregeneration of Chabazite During Nitrification of Centrate from Anaerobically Digested Livestock Waste: Experimental and Modeling Studies, ES&T, 52(7): 4090-4098.
2. Krayzelova, L., Lynn, T.J., Banihani, Q., Bartacek, J., Jenicek, P., Ergas, S.J. (2014) A Tire- Sulfur Hybrid Adsorption Denitrification (T-SHAD) Process for Decentralized Wastewater Treatment, Water Research, 61:191-199.
3. Rodriguez-Gonzalez, L., Miriyala, A., Rice, M., Delgado, D., Marshall, J., Henderson, M., Ghebremichael, K., Mihelcic, J.R., Ergas, SJ. (2020) A Pilot-Scale Hybrid Adsorption Biological Treatment System (HABiTS) for Nitrogen Removal in Onsite Wastewater Treatment, ASCE-J. Sustainable Water in the Built Environment, 6(1): 04019014.

Click images to enlarge in separate window.

Figure 1: Incorporation of HABiTS into onsite wastewater treatment infrastructure.	Figure 2: Conceptual model of the HABiTS process (from Aponte-Morales et al., 2018).
Figure 3: Low cost natural materials used in HABiTS.	Figure 4: Influent NO3- loading (---) and effluent NO3--N (●) concentrations from T-SHAD columns under variable loading rates (from Krayzelova et al., 2014).
Figure 5: Influent and effluent NH4+ -N concentrations during start-up of HABiTS and conventional 2-stage biofilters treating domestic wastewater under variable loads.	Figure 6: Pilot HABiTS with and without recirculation set up at Hillsborough County's Northwest Regional Water Reclamation Facility in Tampa Florida.
Figure 7: Pilot HABiTS schematic. Items shown in grey or with dashed lines were in the system with recirculation only.	Figure 8: Daily NH4+-N concentrations for the Influent (•) and effluent of R-HABiTS 1 (with recirculation ▴) and F-HABiTS 1 (without recirculation ■) for all experimental phases.
Figure 9: Foreground Justine Marshall (MS). Left to right: Dr. Sarina Ergas (PI), Michelle Henderson (MS), Zachary Carroll (BS), Amulya Miriyala (MS), Karl Payne (PhD), Laura Rodriguez-Gonzalez (PhD).	Figure 10: Bench scale reactor set up with and without adsorptive media.
Figure 11: Batch adsorption isotherm studies showing NO3- removal by scrap tire chips.

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