Application of biological treatment device for odors generated at a municipal landfill.

INTRODUCTION
Hydrogen Sulfide (H2S) is a byproduct from anaerobic digestion and when released into the atmosphere it can cause an offensive odor and is dangerous in high concentrations. Processes that produce H2S include wastewater treatment plants, sewers, piggeries, and landfills. Biological odor treatment is a method used to treat offensive odors in an efficient and environmentally friendly method but can be cost-prohibitive for small applications with low concentration. The goal of this project is to lower the cost of biological treatment of odors by introducing an odor concentration step. The concentration step is meant to reduce gas intake volume into the biological treatment system while supplying gas at a higher H2S concentration than produced by the source. The lower gas volume generated reduces the operating cost and size of the biological treatment system. Biological odor treatment has the potential to be more widely used by lowering the operation and installation cost. The test setup of this system is located at the Loup Central Landfill located near Elba, Nebraska. The landfill consists of 7.5 acres that has been filled with 402,000 yards of household waste from 1996 to 2016. An intermediate soil cover was placed on the 7.5-acre area in 2016. There is also a 10-acre area that is currently being filled with household wastes. The project has three main goals: 1) design a three-part biological treatment device, 2) install device at the Loup Central Landfill, and 3) monitor the device's effectiveness at treating H2S.

DESIGN AND OPERATIONAL APPROACH
The key assumption made when designing the system is that the landfill can supply a steady concentration of 1 part per million (ppm) H2S at a flow rate of 2 liters per minute (lpm). The biological treatment system consists of three different steps: (1) collection, (2) concentration, and (3) biological treatment. See Figure 1 for an overview of the biological treatment system steps. Collection System The collection system connects directly to the leachate cleanout system of the Loup Central Landfill. The leachate cleanout system is a network of perforated piping that lies underneath the landfill. There is a total of seven manholes at the surface level that connect to the leachate cleanout system. Figure 2 provides a map of the Loup Central Landfill with cleanout locations. Twenty-five feet of half-inch solid, plastic piping is placed down into the cleanout through one of the seven manholes. Due to the slope of the cleanout pipe, the end of the half-inch piping will sit six feet below the surface. A vacuum pump operating at a constant gas flow rate of 2 lpm is connected to the surface end of the piping. The pump directs H2S laden air into a flow controller. The flow controller directs the flow to the three different columns of the concentrator system. Concentrator System The concentrator system consists of three columns filled with activated carbon that operates in programmed flow-cycle. The cycle for each column consists of a lead phase, lag phase, and desorption phase (See Figure 3). In the lead phase, the flow controller directs the H2S laden gas from the landfill into the first column. The gas slowly saturates the column with H2S. The lag column collects the gas coming out of the lead column and acts as a polishing unit. The gas exiting from the lead phase column is at zero H2S concentration until saturation occurs. Simultaneously, there is a third column going through the desorption phase. During the desorption phase, air passes through the heated column. Studies are still being conducted on different desorption methods. The lead-lag column system consists of two columns in series. The lead column adsorbs most of the H2S while the lag column acts as a polishing tool to safeguard against exhaustion of the lead column. The fluid controller directs the low-H2S concentration gas and desorption fluid to one column at a time. The entrance for each column has a four-way cross to direct the flow from the three different fluid sources. At the exit of each column, there is a solenoid valve to direct the flow to either the next column for the lag phase or to the bio-trickling filter. Figure 4 shows the flow configuration of the concentrator system. The red flow lines indicate low H2S concentration gas coming from the landfill directed to the lead column and subsequently the lag column. The blue lines indicate the desorption fluid sent to the desorption column. The black line directed toward the bio-trickling filter indicates a fluid stream with a high H2S concentration. Biological Treatment System The biological treatment system is a bio-trickling filter. A bio-trickling filter is a column packed with inorganic material that is inoculated with micro-organisms. A polluted gas stream and trickling liquid slowly pass over the packed bed where the micro-organisms digest the pollutants transferred into the liquid stream. The bio-trickling filter is a PVC pipe packed with plastic material inoculated by activated sludge from a local wastewater treatment plant. Concentrated H2S gas is supplied to the bio-trickling filter by the concentrator system. A nearby water tank is pumped at a slow rate into the bio-trickling filter to transfer the H‚‚S into the liquid form. The dimensions of the column are dependent on the flow rate of the H2S gas exiting the concentrator system. The aim is to have an empty bed residence time of 60 seconds or more. Figure 1 provides a diagram of the bio-trickling filter included in the system overview.

RESULTS
The properties of the landfill, the desorption fluid, and the activated carbon directly impact the overall design of the biological treatment system. The main factors that affected are the cycle times in the concentrator system, the size of the concentrator columns, the amount of activated carbon in each column, and the size of the bio-trickling filter. The landfill affects the design based on the H2S concentration extracted out of the leachate cleanout. The H2S concentration is directly proportional to the amount of activated carbon required and the column saturation rates. Figure 5 shows the results of a two-day study on the H2S concentration extracted from the Loup Central Landfill. The gas extracted from the landfill ranged from 0 to 50 ppm H2S during the testing period. Figure 6 shows the range of H2S concentrations extracted from Cleanout #1, #2, and #3. The important characteristics of the activated carbon are the densities, H2S adsorption capacity, and ability to desorb H2S.

SUMMARY
The overall system design is an ongoing continuing project. The system will be functional at the Loup Central Landfill in the summer of 2021.