2009 E3 Small Firms Grand Prize

Onsite Nitrogen Reduction with Two-Stage Biofiltration

Thonotosassa, Florida
ENTRANT: Applied Environmental Technology
ENGINEER IN CHARGE: Daniel P. Smith, Ph.D., P.E., DEE

Fig. 1 Field test system from south showing three parallel two-stage filter systems. Vertical columns are unsaturated media filters (Stage 1) receiving septic tank effluent at 30 minute dose intervals; horizontal columns are anoxic denitrification filters (Stage 2). Upper containment chamber contains multi-head peristaltic pump and repeat cycle dosing timer. The test system was located near a high public use area and resulted in no impairment of recreational amenities.	Fig. 2 Project site at Flatwoods Park, Hillsborough County, a popular public day use facility serving the Tampa Florida area, septic tank effluent originated from single family ranger residence and day use lavatory facilities; test system is to the far right.	Fig. 3 Field test system from northwest showing three parallel two-stage filter systems. Screened openings near the top of vertical columns allowed passive air ingress above the upper surface of the 24 in. media bed to supply molecular oxygen to the microbial communities. Closable sample ports at bottom of vertical (Stage 1) filters allowed sampling of Stage 1 effluent.
Fig. 4 Media deployed in vertical trickle flow unsaturated columns (upper row) and horizontal saturated flow filters (lower row).	Fig. 5 Schematic of Two-State Biofiltration system.	Fig. 6 Denitrification filter medium: left to right, utelite, oyster shell, elemental sulfur.
Fig. 7 Total Nitrogen in STE and Two-Stage Biofilter effluents.	Fig. 8 Total Nitrogen removal efficiency of Two-Stage Biofilters.	Fig. 9 Ammonia nitrogen in unsaturated (Stage 1) filter effluents.

Entrant Profile

Dr. Smith is a President of Applied Environment Technology, a registered Professional Engineer in Florida and Iowa, and a Board Certified Environmental Engineer with the American Academy of Environmental Engineers. He has over thirty years of experience in environmental engineering, and has held positions with two universities, the chemical industry, and state and federal government. Dr. Smith received a PhD in Environmental Engineering and Science from Stanford University, where he conducted fundamental research on anaerobic biological treatment.

Dr. Smith founded Applied Environmental Technology to research and develop new and innovative technologies. Projects have emphasized bench and pilot scale testing of media filtration, membrane bioreactors, and particle separators for onsite sanitation water, stormwater, and other media. Dr. Smith is currently conducting projects in onsite biofiltration of sanitation water for treatment and recovery, hollow fiber bioreactors for urine treatment (with Kennedy Space Center), certification evaluation of stormwater treatment devices, and biological conversion of carbon dioxide for production of renewable energy.

Applied Environmental Technology was the prime contractor to the Florida Department of Health for the Florida Passive Nitrogen Removal Study (PNRS). Dr. Smith led all tasks: literature review, design and conduct of experiments, economic analysis, and final recommendations. Dr. Richard Otis provided assistance in literature review and final recommendations and Mr. Mark Flint in economic analysis. Dr. Smith was solely responsible for all aspects of Onsite Nitrogen Reduction with Two-Stage Biofiltration, including formulating the process concept, reviewing and selecting media, designing and fabricating experimental systems, conducting experiments, monitoring, and performance evaluation.

Project Description

Onsite systems serve over 20% of U.S. households and are a significant component of national water infrastructure. The majority of onsite systems, however, remove a very limited fraction of nitrogen. There is national concern for the potential of septic system nitrogen to migrate to aquifers and surface waters and adversely affect water quality.

Applied Environmental Technology (AET) developed a two-stage biofiltration system that reduces nitrogen in domestic sanitation water by over 97%. The process is simple, robust, and uses only a single dosing pump as sole moving part. Its design was informed by practical experience and the judicious application of fundamental engineering concepts. Two-stage biofilter effluent is comparable in quality to that of large centralized treatment plants. The passive two-stage biofiltration system is an appropriate and sustainable technology for onsite sanitation water treatment that contributes to social and economic advancement and improved quality of life.

AET needed to evaluate a number of complex interacting factors to formulate a nitrogen removal solution appropriate for onsite application. Nitrogen is a multi-species element. Its complete removal requires the combination of sequential biochemical reactions that occur in widely divergent redox environments. The context of onsite treatment requires a resilient system that performs reliably with non-steady flows, varying influent quality, and minimal operation and maintenance. Particulate and colloidal components of influent sanitation water must be retained and steadily degraded to maintain porosity in the filter media for oxygen ingress and to prevent hydraulic failure. Hydraulic application rates must match the assimilative capacity of filtration media. The system should be sustainable with low energy requirement and CO2 emissions.

AET devised an integrated multi-media solution that addresses all of the complex design issues and constraints. A staged biofilter configuration was employed (Figs. 1,2,3), using innovative media (Table 1, Fig. 4) and a dose application protocol. Two-stage filters employed a vertical, unsaturated trickle flow filter (Stage 1) followed by a saturated horizontal Stage 2 filter for denitrification (Fig. 5). The two-stage biofiltration process operates on a single dosing pump as sole mechanical component. Flow from the first filter to the second is by gravity. The biofilters are inherently multi-phase systems, with filter media (solid), water (liquid), and biofilm (organic), and a gas phase in unsaturated filters.

Filter design and media selection were original and innovative. The key to the success was to enable the microbial populations that colonize the filter media to perform their reactions efficiently. Unsaturated filter media (Stage 1) were selected with high water retention to provide long contact time for microbial reactions; high external porosity for sufficient diffusive oxygen ingress to biofilm surfaces, and cation exchange to assist in ammonia retention and nitrification (Table 1). The chosen Stage 1 media had characteristics most critical to achieving low ammonia levels in Stage 1 effluent. A 30 min. dosing interval to the unsaturated filters provided repeated application of small dose volumes and long average retention time for water parcels (Table 2).

The Stage 1 design also employed media size stratification for optimal physical conditions for sequential processes (Table 3). Large size media in the upper level provided pore space for retention of influent particles, hydrolysis, heterotrophic growth, and ammonification. Smaller media sizes in lower filter layers provided large surface area for soluble BOD oxidation and nitrification.

Success of the two-stage design depended on producing a highly nitrified Stage 1 effluent dominated by nitrate and passing it to the Stage 2 filter. Elemental sulfur was provided in saturated (Stage 2) filters as electron donor for autotrophic denitrification:

50 S⁰ + 49.9 NO^3- + 11 CO₂ + 32.8 H₂O --> 2.2 C₅H₇O₂N + 50 SO₄^- + 23.8 N₂ + 50.1 H⁺

Crushed oyster and expanded shale shell were included in the mixed media to supply alkalinity and anion exchange capacity, respectively (Fig. 6). Stage 2 media selection was targeted to produce an effluent with low concentrations of nitrate and nitrite.

Three parallel two-stage biofilters were operated continuously for over eight months on septic tank effluent (Fig. 2). Maintenance consisted only of periodic adjustment of the feed rate. Two-stage biofilters provided exceptional performance and a quality effluent that fully met the treatment objectives and greatly exceeded the expectations of the Florida Department of Health. Total Nitrogen reduction of 97% and Total Inorganic Nitrogen reduction of 99% were achieved (Figs. 7,8, Table 4). Ammonia in Stage 1 effluent and NOx Stage 2 effluent were at or near detection levels, thus validating the fundamental design concept (Figs. 9,10, Table 5). C-BOD5 reduction in Stage 1 filters exceeded 94% (Table 6).

Oxidation Reduction Potential (ORP) was measured in STE and filter effluents to provide insight into operative biochemical processes (Fig. 11). ORP significantly increased in through unsaturated filtration and dramatically declined through denitrification (Fig. 12). Clinoptilolite filter ORP was poised in the region spanning organics oxidation, nitrification and denitrification.

Deposition processes are critical to long term biofiltration performance. None of the Stage 1 media exhibited excessive accumulation of solids (Fig. 13), but did host an expected layer of organic matter (Fig. 14). With the exception of filter entrance regions, no visual accumulation of materials was observed on the on Stage 2 media (Fig. 15). A gelatinous coating was observed at the filter entrances of all three Stage 2 filters (Fig. 16), which ostensibly are biologically unstable regions of sulfur-based oxygen consumption.

Scanning Electron Microscopy images of fresh expanded clay shows numerous 10 to 100 um surface depressions that contrast with the smooth surface appearance after 246 days (Fig. 17). Decrease in surface roughness is consistent with the operating principal of unsaturated aerobic filters: accumulation of particulate and colloidal components of sanitation water, polysaccharides, and other microbial exudates, and microbial synthesis. In contrast, the roughness of the sulfur surface increased through treatment (Fig. 18). This is consistent with slow sulfur dissolution, which is the operating principal of sulfur-based denitrification filters. An Electron Dispersive Spectrometry scan of the elemental sulfur particle surface (Fig. 18b) showed the expected domination of elemental mass by sulfur, even after the eight month deployment.

Fig. 10 Total oxidized nitrogen in saturated (Stage 2) filter effluents.	Fig. 11 Monitor final effluent water quality: Luminescent Dissolved Oxygen (LDO), Oxidation Reduction Potential (ORP), and pH probes were placed directly at the efflux surface of denitrification filter media (i.e. the Stage 2, or final effluent) before contact with the atmosphere.	Fig. 12 ORP profile through Two-Stage Biofilter (Clinoptilolite/75% Sulfur).
Fig. 13 Upper surface of clinoptilolite media following 246 days of continuous operation on septic tank effluent. Media appearance was similar to unused media, with clearly discernable individual grain geometries. The visual absence of excessive organic material suggests a low propensity for clogging and hydraulic failure over longer term operation.	Fig. 14 Enlarged upper surface of clinoptilolite media following 246 days of continuous operation on septic tank effluent. A moist and gelatinous coating is visible on individual grains, which is an expected characteristic of biofilm action. Successful long term operation requires that a balance be maintained between the sorption of particulate and colloidal components and microbial growth, and biodegradation and cell decay.	Fig. 15 Denitrification media appearance after 246 days of operation at 21 in. distance from entrance end in 60/20/20% sulfur/oyster shell/utelite column. Visual appearance was similar to unused media with no visually discernable materials accumulation.
Fig. 16 Denitrification media appearance at entrance end of 75/25% sulfur/oyster shell column after 246 days of operation. Discernable material accumulation was observed on media on the right side of photograph, which was in the biologically unstable entrance region.	Fig. 17 SEM image of expanded clay surface. a.) fresh media, b. Filter 1B, 1 to 2 in. depth.	Fig. 18 SEM image of elemental sulfur surface a.) fresh media, b. Filter 2A, 21 in. from entrance.
Fig. 19 Electron microbeam/electron dispersive spectrometry scan of elemental sulfur surface from previous SEM image, showing expected domination by S.