Disposal of Swine Lagoon Sludge with Nutrient Recovery

By M. T. See, J. Koger and P. Burnette

 

North Carolina State University Swine Manure Management Research from 2004-2005. Sludge accumulation in lagoons reduces their treatment efficiency over time and, ultimately, may cause the decommissioning of the lagoon. Currently, much of the swine industry is dependent upon lagoons for waste treatment and storage prior to land application of the effluent. Newer regulations in some areas require no less than 4 feet of treatment depth in the lagoon. Thus, sludge removal is required more frequently. Disposal of sludge by a means other than application to local spray fields would eliminate the threat of P over-application to cropland. Gasification is the ideal technology for this disposal since the valuable nutrients are recovered in usable form and the mass of material is greatly reduced (estimated at 50 fold) making hauling to sites of need economically feasible. Furthermore, gasification is a clean technology lacking the emission concerns of incineration. Processed ash can be a fertilizer constituent or concrete amendment. However, the optimal form of recycling would be to reuse the minerals in animal diets. Preliminary digestibility trials have shown ash P is highly digestible, making this an attractive, zero waste end use.

 

Materials and Methods

 

Twelve kilograms of sludge were sampled from each of 20 different swine lagoons (8 finishing sites, 6 nursery sites, and 6 sow farm sites). Descriptive data on the lagoons at the time of sampling is in Table 1. The selected farm sites were visited one time to determine the lagoon dimensions, lagoon depth (supernatant and sludge) and the depth of sludge accumulation. The total depth of the lagoon was measured by dropping a 4.5-kg weight, which penetrated the sludge and rested on the lagoon bottom. The depth of sludge was measured by sinking a non-weighted 20-cm diameter Secchi disc, without a weight, to settle on the sludge-liquid interface. Sludge samples were taken randomly within the lagoon. Three sludge samples were collected along the midline of each lagoon. Samples were collected with a Standard Ekman Grab sampler (3450-ml). The sludge sampler was allowed to settle into the sludge layer, then a trigger weight was dropped to close the “jaws” of the sampler. The three samples were mixed together and a composite sample was collected for laboratory analysis.

 

The dewatered lagoon sludge samples were combined and processed into a 300 kg sample for gasification of the dewatered sludge to evaluate the conditions required for recovery of ash with little carbon contamination and with minimal energy expenditure. Dry matter determinations were by passive drying in a 60°C oven until the weight loss in 24 h was less than 1% of the previous day’s weight. Alternatively, samples were dried in a Heto Power Dry LL3000 freeze dryer to preserve sample energy content. Samples were then processed in a Retsch ZM 100 (Newtown, PA) grinder equipped with a 0.5 mm screen. The energy content of feedstocks and residuals was determined by bomb calorimetry in an IKA Werke C5003 (Wilmington, NC) instrument. Each sub-sample was analyzed twice if the results were within 50 calories per gram of each other. If not, additional sub-samples were analyzed until replicates meeting this criterion were obtained. Mineral analysis of sludge samples was by a Vista MPX Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES) manufactured by Varian (Palo Alto, CA). Samples were prepared based on AOAC digestion method 975.03. Briefly, approximately 1 g of sample was weighed into a quartz crucible and ashed over night in a muffle furnace at 500ºC. Then 15 mL of HNO3 was added and evaporated to dryness on a hot block. Samples were returned to the muffle furnace over night at 500ºC. Then 7.5 mL of HCl and 7.5 mL of HNO3 were added and samples were heated to 95ºC for about 2 hours. Samples were then brought up to 50 mL with deionized water and filtered before analysis. Carbon and nitrogen determinations were made on a Thermo Finnigan soil analyzer, Flash EA 112 Series (Thermo Electron Corp., Boston, MA).

 

During trials, operating parameters were monitored at 15-minute intervals. Dwyer pitot tubes, 5/16 inch outer diameter, provided differential pressure information for calculation of flue flow rates. The pitot tube was placed in the center of the duct and pressure information was provided by Dywer gauges, model 2000-00. The equation used to determine flow through the flue was: V_A X A_D = F

 

where V_A is the air velocity within the flue, A_D is the cross-sectional area of the duct, and F is the flow rate expressed in ft³/min. Air velocity is calculated as 1096.2 times the square root of P_V/D where P_V is the velocity pressure in inches of water and D is the air density in lb/ft³. Air density is calculated as 1.325 times the barometric pressure (inches of mercury) divided by R, the absolute temperature, which is equal to the temperature in °F plus 460. Type K thermocouples were placed within the primary, secondary, and flue chambers in order to log temperature data at 60 second intervals throughout the gasification process. In addition, the control panel provided constant read-out on temperatures in the primary and secondary chambers. Finally, a Kane-May analyzer (Kane International Limited, Hertfordshire, GB), model number 9106 or 900, was used to monitor NO_x, SO₂, CO, CO₂, O₂, and temperature at 15 minute intervals. When possible, hydrocarbons were also measured with the Kane-May 9106 instrument. Propane consumption was recorded from an in-line meter with sensitivity to the 0.1ft³ level. Tests were conducted to define the throughput time, optimal operating temperature, energy consumption, and loading procedures to minimize heat loss.

 

The Brookes Gasification Process (BGP) gasifier, a product of Infectrol (Scarborough, Ontario, Canada), was originally developed for disposal of hazardous hospital waste. Thus, in its development, completeness of disposal and absence of polluting emissions drove the design. The simple, “fool-proof” design make it attractive for disposal of animal waste where low cost, ease of operation, disposal of waste, and recovery of a safe nutrient-ash product are the principal considerations.

 

The BGP batch system is a fixed bed, indirectly heated, batch-fed gasifier suitable for processing a wide range of feedstock materials (Figure 1). The unit has two chambers: a lower burner chamber, referred to as the secondary chamber (SC), and an upper gasification or primary chamber (PC). Hot gases from the burner heat the refractory bricks lining the secondary and primary chambers. Feedstock sits undisturbed on the hearth of the PC where it is indirectly heated to 600 – 1000 ºC (1100 – 1800 ºF) by heat transfer through the refractory bricks. As the feedstock is broken down, or cracked, into low molecular weight gases and volatile hydrocarbons, these products are drawn into the burner chamber through a small opening at the top of the back wall (Figure 2). Energy from the out-gases provide fuel to sustain the process. At startup, the SC is heated using a fuel such as propane or product gas from another unit (commercially, multiple units could be operated on one site). Both temperature and oxygen availability can be controlled during the reaction. This control and the static nature of the system result in a very clean process that meets the emission standards both in Europe and California without requiring gas cleanup. The ash consists almost exclusively of mineral compounds with virtually no carbon char remaining. Since product gas is combusted, the energy potential of the system is limited to capturing waste heat for hot water generation, steam generation, or electrical generation. The BGP gasifier used for these studies has overall dimensions of 6 feet in length, 5 feet in width, and 5 feet in height. The gasifier has a mild steel outer shell, which serves as structural support as well as protection from the environment. The next layer is a high-grade ceramic insulation, which can handle temperatures up to 3000 F. The inner layer is the refractory firebrick. The hearth of the primary chamber (PC) is a high temperature ceramic tile floor. The dimensions of the PC are 42 inches deep, 47 inches wide, and 22 inches in height. In the back left top corner of the PC, there is an opening through which the gases are drafted into the SC (Figure 2). The opening is approximately one square foot, but can be reduced in size by the addition of refractory brick. The door to the PC is approximately 4 inches larger in both width and height than the PC. The door is a mild steel shell, which is packed with ceramic insulation. There are three air doors (AD), or ports evenly spaced along the width of the door and located approximately one-third of the way up the door from the bottom, each is 2.5 inches in diameter. These AD’s allow less than stoichiometric air, or the air required to fully oxidize all the carbon, to enter the PC during the “carbon cycle” in order to “bleach” the carbon from the ash. These holes have plates that can be adjusted to allow different rates of air entry.

 

Results

 

Lagoon sludge samples ranged in dry matter from 5.9 to 11.7% but did not differ by farm type (Table 2). Energy content was 21.5% lower for lagoon sludge sampled from sow farms as compared to sludge samples from nursery and finisher lagoons. However, sludge from sow farms had the greatest concentration of calcium.

 

Sludge sampled from nursery lagoons showed higher concentrations of sulfur, iron, manganese, copper and zinc than sludge form finisher or sow farms (Table 2). Sludge from finisher lagoons possessed higher concentrations of potassium and carbon (Table 2).

 

Older and larger lagoons had lower concentrations of carbon and organic nitrogen (OR-N) in the sludge layer (C_area = -34.4 lb/1000 gal/acre; C_age = -4.96 lb/1000 gal/year of age; OR-N_area = -3.85 lb/1000 gal/acre; OR-N_age = -0.40 lb/1000 gal/year of age; P < 0.05) than newer and smaller lagoons.

 

The batch gasifier was slightly modified to accommodate the wet material. Firebricks were used to create a holding chamber in the PC of the batch gasifier. Three chambers were laid out in the PC in an attempt to help with the efficiency. The waste was pumped into the three chambers at different time intervals, similar to pistons firing in a car’s engine (Table 3). The lagoon sludge was successfully gasified and the ash was completely processed without any carbon left in the ash residue. However, because of the low dry matter the result was poor efficiency. It took 196 cubic feet or 5.39 gallons of propane to gasify 180 lbs of the lagoon sludge at 15.4 % dry matter (1.08 cubic feet propane/lb sludge or 0.03 gal. Propane/lb sludge at 15.4% DM). Sludge was processed through the BGP gasifier at a rate of 1.28 lb/minute.

 

A spreadsheet was developed in order to calculate the amount of propane needed to operate the gasifier with different feed rates and different DM’s. Table 4. is the result of these calculations. It was assumed that one could double the feed rate of the gasifier if the PC chamber was split into an upper and lower chamber without affecting the amount of fuel used to sustain the gasifier without a feedstock. If lagoon sludge can be dried to 55% dry matter than the existing gasifier using only one hearth in the PC will allow the gasification of the sludge with no propane needed. However, if the feed rate can be doubled by adding a second hearth in the PC than the needed DM can be reduced to 38%, which is more feasible than drying the waste to a high level of 55%. While the batch gasifier was used to calculate this data, the continuous feed gasifier would be a better choice because of its improved efficiency over the batch system.

 

Advantages of the continuous feed BGP gasifier over a batch gasifier design are: 1) better use of waste heat throughout the system, 2) potential of forming a higher value product gas suitable for catalytic conversion to liquid fuels or the chemical building blocks of plastics (replacing fossil sources), and 3) waste heat recovery from the flue. These should improve the overall economics of the gasification technology as a waste treatment methodology.

 

The amount of man-hours spent in operation and maintenance of the BGP gasifier depends on the level of automation built into the machine. For research purposes, full manual control was desired in order to have maximum flexibility in operating conditions. However, the burner, secondary air, air doors, damper, and temperature control can all be automated without much added expense. The addition of an automated loading/unloading system for a batch-fed unit would increase the price substantially, but it would reduce the operator time to nearly zero.

 

It has been previously demonstrated that the manual operation of the unit requires few man-hours when feedstock loads are consistent in amount, DM% (within 15%), and type. Under these assumptions and with the unit operating 24 h/d, the man-hours for a 24 hour period, including loading and unloading, would be a total of <4 hours. The machine would need maintenance about twice a year if it were operated within normal operating parameters. Simple spot inspections of the firebrick and the PC door’s insulation would be needed on a weekly basis. This could be done without turning off the unit. The firebrick is very brittle once it has been cured so that when loading and unloading the feedstock and ash care must be used not to damage these components. If undisturbed the firebrick can last 10 years. The firebrick and the hearth can be patched if either is damaged. The PC door’s insulation will see the most abuse from opening and closing of the door, and therefore will require the replacement of part or all of the insulation once a year. The machine will have to be cooled down to ambient temperatures before the insulation or firebrick can be replaced or patched. The burners will need to be inspected 2-3 times a year for wear of the ignition electrodes and the flame sensor. This can be done with the unit still hot, but the power and propane must be turned off. Both electrodes can be replaced within 1-2 hours if they have failed. No other maintenance issues have arisen beyond the burner electrodes, the occasional cracks in the firebrick, or the partial replacement of the PC door insulation.

 

Operation and maintenance of the BGP gasifier depends on the size of the unit in question. Capacity for feedstocks varies according to properties of the feedstock such as percent ash and percent DM. BGP units can also be grouped together or scaled up to meet the demands of the application.

 

Discussion

 

In general, swine lagoon sludge is a high ash and low energy material. Its composition is influenced by the age of the lagoon, its loading rate, the ration ingredients, and the type of swine operation being served, for example: farrow to finish, farrow to feeder, or grow-finish only (Miner et al., 1983; Bicudo et al., 1999). With grow-finish and farrow to finish operations, the sludge is 7-13% solids (Miner et al., 1983; King, 1981) and contains 0.4% each, N, P, Ca, and Mg, and 0.07% K (Bicudo et al., 1999 ; Barth and Kroes, 1985). This high P content, however, makes it a poor fertilizer with a high risk of Psaturation of soils.

 

The high moisture content of sludge makes long-distance hauling for land application cost prohibitive. Sludge mass can be reduced by dewatering methods such as centrifugation, thereby increasing the solids concentration to as much as 30%, with solids recovery ranging from 60 to 99% (Miner et al., 1983). This can greatly improve handling properties, but additional weight reduction is still necessary for cost-effective hauling.

 

The mineral composition of the sludge is substantially different from that of fresh waste as nitrogen is preferentially partitioned in the lagoon supernatant (70% of total N) from where it escapes in gas form (Bicudo et al., 1999). In contrast, P is concentrated in the sludge (90% of total P, Bicudo et al., 1999). Thus, the N: P ratio of pit-collected waste is 4:1 (Luo et al., 2002). However, in lagoon sludge the ratio is approximately 1:1 (Bicudo et al., 1999) and in processed, pelletted lagoon sludge, the ratio is 0.68: 1 (Duffera et al., 1999). This relatively high P and Ca and low N and K content, however, makes the minerals in sludge an interesting feed P source.

 

It is known that antibiotic residues, prions, and some parasite eggs can survive lagoon conditions and some processing methods, such as anaerobic digestion (Johnson et al., 1998), so the sludge processing method selected must destroy these if use as feed additive is to be considered. One method to recover concentrated mineral nutrients and simultaneously destroy any bioactive contaminants, is to gasify the sludge.

 

Gasification offers many advantages for processing undesirable waste materials. Tests on gasifier emissions reveal very low levels of NO_x, SO_x and volatile organic carbons in the flue gas, well within emission guidelines. The only products, typically, are a combustible gas and a sterile mineral ash. The process occurs at temperatures in excess of 600 °C (~1100 °F) in an oxygen-starved environment. Some gasifiers cannot efficiently process such high moisture feedstocks as sludge. The Brookes gasifier, however, is able to handle up to 70% moisture. Although driving off the moisture incurs an energy “penalty,” this can be diminished by using waste heat from the unit to further dry the feedstock prior to loading it into the reaction chamber.

 

The Brookes design uses a batch-feed approach to indirectly heat the biomass, combust the product gases to sustain the reaction and reduce operating costs, and recover a sterile ash that has minimal carbon contamination. The small amount of nitrogen remaining in the sludge is converted to N₂ gas. Not only is the Brookes gasifier ideally suited to this application, it is versatile enough to function as a mobile unit that can move from site to site for processing the sludge, thereby minimizing transportation costs. Another major benefit is that the design is very simple. An ‘of the shelf’ blower and door are the only moving parts. Thus, it can be easily maintained and operated by a person after only minimal training.

 

Conclusion

 

Gasification offers many advantages for processing undesirable waste materials. Tests on gasifier emissions reveal very low levels of NO_x, SO_x and volatile organic carbons in the flue gas, well within emission guidelines. The only products, typically, are a combustible gas and a sterile mineral ash. The key to gasification of lagoon sludge will be the Dry Matter level. If the swine lagoon sludge can be successfully dried to an appropriate level before it enters the gasifier using the gasifier’s waste heat, than the system will be self sustaining and possibly, it could have a net energy balance to produce hot water, steam, or electricity.

 

Table 1. Lagoon Descriptive Data.

 

Table 2. Nutrient composition of lagoon sludge by farm type.

 

Table 3. Sludge (15.4% DM) processing time and sequence in a Brooks Gasifier.

 

Table 4. Estimated processing requirements for lagoon sludge at differing dry mater levels.