THERMOPHILIC DIGESTION PLANT
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Introduction
The Moorhead Group has recently acquired the marketing and licensing rights for a proprietary and highly efficient process to convert biodegradable waste into useful agricultural products. The process and equipment was first proven on a pilot plant scale and is now in full-scale commercial operation. A modular system has been developed to accommodate commercial facilities of any size.
The process was adapted from the Autogenous Thermophilic Aerobic Digestion (ATAD) process that is rapidly replacing other methods for digesting of municipal sewage sludges. Adaptations have broadened the ability of the process to handle the wide range of biodegradables commonly encountered in municipal wastes. Biodegradables from leafy plants (i.e. vegetables) can be converted to a stable useful product in approximately 36 to 48 hours, while biodegradables from woody sources, including paper waste, require processing times ranging from 48 hours to 100 hours. The process permits the feeding of a mixture of raw materials of variable digestibility. The process allows for separation of hard to digest material for extended treatment. Non-biodegradables such as metals, glass, sand, and plastics are segregated by a unique infeed system for recycling, for disposal to landfill, or removed by cleaning and screening during the liquid phase process.
The commercial scale plant processes 55 tons per day of municipal biodegradable wastes containing up to twenty- percent wood fiber-based materials including cardboard and paper. Products from the plant include a granular organic soil enhancer with an average NPK analysis of 6-1-1, and liquids with high concentrations of micro-nutrients which are used as the organic base to prepare liquid fertilizer preparations for hydroponics growers and for spray applications to lawns and gardens. During seasonal periods with low demand for spraying, the liquids are concentrated to become part of the solid granular product.
The plant uses 25, 35 and 65 metric ton digesters. Experience has determined that 65 ton is the most practical size for primary and secondary digesters. Specially designed ‘reject digesters’, used for extended treatment, continue to be most appropriately sized at 25 metric tons. Facility plans have been developed using 65 and 25 ton digester modules to satisfy the plant capacity requirements of 400 and 550 tons per day. The aeration equipment for the digesters was developed in-house for the application and is patented. The digestion process can not be commercially operated without the use of this patented equipment.
The relatively small space requirements and the ability to control odor emissions permit locating the thermophilic digestion facilities in urban settings close to the sources of prime biodegradable waste materials and potential customers for marketing end products. Less hauling of waste equates to lower waste handling costs. Diversion of biodegradables from landfill sites would eliminate current odor and leachate problems and the remaining waste could be subject to other recycling programs.
A metropolitan community generates approximately two pounds of biodegradable waste per person, per day. Growers, wholesalers and retailers of vegetables generate approximately one-third of this waste. Food recycling programs in concentrated food processing areas have been known to divert as much as 750,000 tons per year from landfill. One-sixth is generated in the form of leftovers by restaurants. Households generate the remainder. Without residential curbside recycling a plant can expect one pound of biodegradable waste per person / per day, but with curbside recycling this content can double. The plant currently charges $10 less than the current landfill tipping fees to attract waste haulers. The plant capacity has always been filled immediately at $30 per ton. This amount will vary dependent upon geographical location. The centralized location of the plant substantially reduces hauling time.
On average, green biodegradables contain between 70 and 90 percent water, while paper products contain only 10 percent water. The mix of green biodegradables and paper wastes can therefore affect the water/solid balance in the plant. The solid products are dried to 5 percent water content. For each ton of solid product, slightly more than 1 ton of water is evaporated in the drying process. One ton (2200 pounds) of raw material input on average results in the following products (in pounds):
Source green waste paper waste 80/20 mix
other recyclables and waste 209 11 220
solid fertilizer 191 407 598
water vapor to atmosphere 595 22 607
liquid fertilizer 775 0 775
1760 440 2200
As can be seen, running a higher paper content can accommodate for reduced seasonal requirements for liquid fertilizer, however, this also results in a slight reduction in plant intake capacity.
The lawn fertilizer retails for $14 per 20-lb. package. Bulk fertilizer sells to producers and large volume end users at wholesale prices of $ 230 per ton. Bulk liquid fertilizer sells to lawn care companies for $ 130 per ton. Retail sales are expected to increase as consumers recognize the superior benefits of organic fertilizers and reduced environmental impact from recycling. Effective marketing will promote the products and enhance their full retail value.
The series of granular solid products have proven to be high-grade organic commercial fertilizers. A research program was designed to determine the effect of these fertilizers on the growth of Pencross bent grass sod. The treatments were also designed to be a guide for the potential field application rates. Solid Organic Fertilizer (6-1-1) and chemically enhanced Liquid Organic Soil Restorer (Compost Concentrate 6-4-5) were the two products applied repeatedly at three week intervals to pots of turf being grown in a typical greenhouse environment. Two conventional chemical fertilizers were also included for comparison purposes. The application rate used for the commercial fertilizers was based on delivering one lb. of nitrogen per 1000 sq. feet for each application period as per the manufacturer’s specifications. The organic products, in addition to being tested at this level, were also tested at one-half and one-quarter of these levels. Fertilizer was applied at three-week intervals for a total of fifteen weeks. All the grass produced was harvested prior to each application of fertilizer.
Both the Solid Organic Fertilizer and the Liquid Organic Soil Restorer increased the fresh and dry weight of clippings taken when compared to turf that received only water. The classical response to various concentrations of fertilizer was observed for both organic products. In addition, both of these fertilizers produced fresh and dry weights that were comparable to or better than two leading brand commercial fertilizers with the following N:P:K levels (10:18:18) and (30:10:10). In fact, for several harvests only one-half or one-quarter of the Solid Organic Fertilizer were required to produce a similar response to the commercial fertilizer. Furthermore, repeated applications of both the solid and liquid organic products did not result in a toxic effect, which appeared in the treatment with one of the chemical fertilizers. The chemical fertilizer toxic effect was expressed in a general yellowing and decline in the amount of grass produced (see graph).
Initial marketing plans for the organic fertilizers is to focus on bulk sales to lawn maintenance companies, organic farmers, hydroponics growers, golf courses, and municipalities. A marketing agreement has been reached for retail sales as an organic lawn fertilizer. Consumer demand should increase with the awareness that organic fertilizers derived from biodegradable recycling are superior to chemical fertilizers of higher N:P:K value. Organic fertilizers can very profitably be marketed at lower cost than chemical counter-parts. Moreover, usage shows that less organic fertilizer is required for better results and the price will ultimately be determined by market demand.
A modular system of digestion capacity has been developed. The process occurs at six- percent solids, which equates to digester capacity of 10/6 or 1.67 tons to process one ton of green waste. Three days of processing time, plus 10 percent for inoculation = 1.67 x 3 x 1.1 = 5.5 tons of digester space per ton of green waste. A 150 short ton clean waste plant therefore requires (150 x 5.5) (60 x 1.1) equals a total of 12 primary and secondary digesters and two 20 ton reject digesters. Space requirements for this 14-digester size plant (150 short tons of green waste) include:
26,000 sq.ft. building space
7,500 sq.ft. outside tankage space area
30,000 sq.ft. load and yard area
This space requirement includes room for:
· unloading and storage
· processing
· bagging
· warehousing
· scrubbing
· office space
· shop space
· loading area
The attached sketches illustrate a simplified process and plant layout. For plants with larger feedstock capacity the digesting space requirement can be directly adjusted to capacity. Other space can be prorated on a square foot factor basis.
The plant discharges air that contains regulated pollutants as well as potentially odorous gasses. All off-gasses including plant air changes are collected for purposes of scrubbing in a biofilter backed up by water scrubbers. Extended operation has demonstrated to the satisfaction of regulatory authorities and close neighbors that a properly operated plant (all portions are kept aerobic) can be kept well within allowable emission levels.
Environmental and Permit Issues
The plant operates under a closely monitored environmental permit. The regulatory agency responsible for the monitoring of all aspects of permit conditions has determined complete compatibility with industrial neighbors and residences within a 500-ft. radius and has issued standard permits to operate. Operational experience have extensively demonstrated to neighboring residents within the municipal district that the plant can be controlled to maintain full air quality compliance at all times.
The greatest source of possible nuisance is the transport of raw materials to the plant. At the plant, trucks are unloaded in an environmentally controlled building that maintains a slight negative pressure. Air-for-air changes are scrubbed to eliminate odors using biofilters. Incoming material is sorted and slurried as quickly as possible and pumped to the enclosed digester vessels. Filters used for building air discharge control scrub off-gasses from the aerobic digesters.
The permits to operate a commercial scale facility allow the operation of a 130 metric tons per day infeed capacity, appropriate for the location. No difficulty should arise in obtaining permits in other municipal jurisdictions for any size plant.
The Pilot Plant: Thermophilic Aerobic Digestion of Food Waste
This report on the operation of the pilot plant was published in August 1994:
The pilot plant for conversion of vegetable and fruit waste to value added organic products have been in operation for almost 1 year. A thermophilic aerobic digestion process is employed for the waste conversion.
This report describes the operation during a typical batch run and presents the results of the liquid and dried products as fertilizers or soil additives. The test was conducted at the pilot plant during the period between July 4 and July 11, 1994.
Over a period of three days, a total of 40 tons of vegetable and fruit waste was delivered to the pilot plant. The waste was processed and used for thermophilic aerobic digestion. The waste contained approximately 85% vegetables and fruit matter and 15% non-vegetable matter. Among the non-vegetable matter, approximately 70% were large objects such as waxed and non-waxed cardboard and wooden crates. The remaining 30% of the non-biodegradable contaminants were small objects such as plastic bags, rubber bands, twist ties, fruit baskets, rubber gloves, bottle caps, staples, metals, etc. Prior to substrate preparation the large non-vegetable objects were removed manually. To process 14 tons of waste per hour, one front-end loader and two men were required, using a hand-sorting procedure. More tipping floor space will be required if the company expands its operation to process up to 100 tons of waste per day. The small contaminants were removed by mechanical means prior to and during digestion. The digestion process is capable of removing contaminants efficiently from waste containing 25% non-vegetable matter. The removal of small non-vegetable contaminants from the slurry has made possible the production of unusually clean end products free of metals, plastics and glass.
To optimize the substrate for thermophilic aerobic digestion, the particle size of the waste was reduced and the waste was diluted with the filtrate generated from the previous batch digestion. Thus, the liquid used in the process was recycled. No process effluent was discharged from the plant. Throughout the size reduction process operating parameters such as the pH, particle size and slurry consistency was monitored continuously. The consistency of the slurry was controlled by the amount of filtrate added to the vegetable matter. Suitable slurry consistency was made according to the handling capacity of the slurry transfer pumps. The size reduction equipment was capable of processing 40 tons of material in three hours. Approximately 1/2 hour was needed to clean and dispose off non-vegetable matter. At present, the size reduction unit is capable of processing 100 tons of waste per day.
Results of chemical analyses showed that the substrate contained 5.8% total solids, 3.5% total volatile solids and 94.2% moisture content. The dried matter consisted of 344 g BOD/Kg, 711 g COD Kg, 21.1 g N/Kg, as TKN, 5.5 g N/Kg as ammonia, 2.5 g P/Kg as total phosphorous, 2 g P/Kg as ortho-phosphorous and 48.8 g k/Kg as total potassium (Table 1).
The slurry substrate was digested aerobically in closed vessels. To enhance aerobic oxidation two stages of digesters were used. Air was introduced into the digesters at a predetermined rate to create an aerobic environment for bacterial growth. In addition, the waste slurry in the digester was mechanically agitated to provide sufficient mixing and surface contacts together with readily available oxygen for optimum digesting of the biodegradable organic material. To accelerate the digestion process, the starting temperature of the slurry was raised to the usual thermophilic digestion temperature range. Once the population of thermophilic bacteria started to increase rapidly, the temperature was maintained by the heat generated from bacterial metabolism. Throughout the digestion process the temperature, pH, air flow and slurry substrate volume were monitored and recorded.
The slurry substrate was first digested for 48 h in a 60-ton primary digester. The partially digested slurry was transferred into two 30-ton secondary digesters and incubated further for another 24 h. The company was equipped with one 60-ton primary digester and two 30-ton secondary digesters capable of processing 40 tons of food waste easily every four days. For full-scale commercial production seven primary and five secondary digesters will be required.
After digesting the slurry for a total of 72 hours, it was dewatered in a filter press, dried, screened, and stored. The filtrate from the press was clarified and stored. During the process, the slurry, dried product and filtrate were analyzed over a five-day period for biochemical oxygen demand (BOD), chemical oxygen demand (COD), total solids (TS), total volatile solids (TVS), total Kjedahl nitrogen (TKN), nitrate, ammonia, potassium, orthophosphate and total phosphorous.
The primary digester removed 16% BOD after 24 hours, 35% after 36 hours and 39% after 48 hours (Table 2 and Figure 1). Further BOD removal was demonstrated after secondary digestion resulting in a final BOD removal of up to 62%. These results show that organic material was readily biodegraded throughout the digestion process. Also, up to 37% volatile solids were removed (Table 3 and Table 2).
The digested organic products were separated into solid and liquid portions. Results showed that high fertilizer values were found in both solid and liquid products (Table 4). A portion of the end product was enriched with cardboard and nutrients. Samples of the end product with and without cardboard and nutrient addition were tested for their fertilizer value. Results indicated non-enriched solid product was found to be comparable or even contained higher nitrogen, phosphorous and potassium values than commercial organic fertilizers (Table 4).
The non-enriched solid product (without cardboard) contained approximately 4.1% nitrogen and the enriched material 8.3% nitrogen. Total phosphorous and potassium levels did not increase significantly by enrichment. The non-enriched liquid product contained only 0.02% nitrogen and 0.3% potassium and non-detectable levels of phosphorous. When the liquid product was enriched with nutrients an increase of up to 3% nitrogen, 0.02% phosphorous and 1.2% potassium were detected. Heavy metals were not present in the end products at concentrations that are considered to be harmful to plants (Table 5). In fact they were significantly lower than those in sewage sludge composts and fertilizers.
An outstanding feature of the digested organic end product is its clean fertilizer appearance. No foreign objects such as glass, metal chips, or rubber bands can be seen. This is important for a product to be used as fertilizer or soil additive, because the presence of these objects may be harmful to the soil.
Based on the preliminary results of this study, the following conclusions and recommendations for future studies may be drawn:
· The thermophilic aerobic digestion process has successfully produced a clean totally organic solid and liquid product with high fertilizer values.
· Biological metabolic activity during digestion has raised the temperature of the fermenting slurry and maintained high temperature throughout the fermentation process that kills pathogens and weed seeds.
· Nutrients of the original plant materials have been recovered.
· Destruction of disease causing microorganisms and weed seeds and the recovery of nutrients are not the only advantages of this unique digestion process for recycling of food wastes. Many agronomists believe that plants grown in organic fertilizer are healthier and produce sweeter fruits than those grown with chemical fertilizer. This is most probably due to the presence of micronutrients. These trace metals are not present in conventional fertilizers.
· The organic fertilizer products improve the soil structure, increase water-holding capacity, improve aeration of soil, and enhance soil cultivation and root penetration.
· Most plant growth media are made from peat to which are often added sand or other bulking agents. Addition of nitrogen and other inorganic nutrients including phosphorous, potassium and trace elements promote germination and root growth. The material should have a good water retention capacity, adequate porosity, good drainage, and stable structure. Medium grade sphagnum peat is used commercially but is currently becoming increasingly scarce. Therefore, an inexpensive, easily produced material which possesses the beneficial properties of peat would be an ideal substitute. The end products possess these properties.
There are a number of potential markets for the organic fertilizer products, including organic growers who presently have limited supply of organic seedling and potting soils. The products may also be used extensively as tree or shrub planting medium or as an all-purpose plant growth medium. Other potential uses of the product include animal and fish feed-additives. Current research is emphasizing higher value end-product recovery (i.e. Vitamin B12, hormones, and industrial enzymes) to increase the amount of value-added components.
Table 1: Characteristics of Substrate Feed
Parameter Substrate
pH 6.5
Total Solids (%) 5.9
Total Volatile Solids (%) 3.6
Moisture 94.2
BOD (9/Kg)* 344
COD (g/Kg)* 711
TKN (g/Kg)* 21.1
NH3 (g N/Kg)* 5.5
Total P (g P/Kg)* 2.5
P (g P/Kg)* 2
Total K (g K/Kg)* 49.9
*Based on dry weight
Table 2: BOD Removal Profiles at Various Digestion Times
Time (h) BOD (Kg) BOD Removal (%)
Start 892 0
12 896 0
24 753 16
36 576 35
48 557 39
72 338 62
Table 4: Comparative Fertilizer Value End Product Organic Fertilizer, Commercial Composts, and Commercial Organic Fertilizers
Type of Fertilizers N P K
(%) (%) (%)
End Product (no cardboard; no nutrients) * 4.1 1.0 1.5
End Product (no cardboard; + nutrients) * 8.3 1.2 1.4
End Product (+ cardboard; and nutrients) * 8.7 0.7 0.8
End Product (+ cardboard; no nutrients) 5.4 1.4 2.0
Clarified Filtrate (no nutrients) 0.02 <0.001 0.3
Clarified Filtrate (+ nutrients) 3.0 0.02 1.2
Wood Chips/Raw Sewage 1.0 N/A 0.2
Municipal Waste/Digested Raw Sludge 0.8 0.03 0.4
Bedding Materials/Cow Manure 1.0 1.0 1.0
Worm Castings 1.6 0.1 0.02
Bone Meal/Dried Blood 5 5 5
Dried Sewage 6 2 0
* Based on g/Kg dry weight
** Based on g/Kg wet weight
N/A Not Available
Table 5: Comparative Heavy Metal Composition of Organic Fertilizer and Sewerage Sludge Organic Fertilizers
Name Ar Cd Cr Cu Ni Pb Zn
End Product (no <3 <1.4 5 5 4 6 41
cardboard; no nutrients)
End Product (no <3 <1.3 6 4 5 6 37
cardboard; + nutrients) *
End Product (+ <2 <1.1 2 2 1.6 <2.1 19
cardboard; and nutrients) *
End Product (+ <3 <1.5 4 4 4 <3.1 33
cardboard; no nutrients) *
Clarified Filtrate (no <0.2 <O.1 <O.1 <O.1 <0.1 <0.2 0.1
nutrient)
Clarified Filtrate (+ <0.2 <O.1 <O.1 <O.1 0.5 <0.2 0.7
nutrient)
Sewage Sludge Composts N/A 8 N/A 300 5.5 290 770
Sewage Sludge Organic <33 35 5,000 390 141 276 1,000
US. EPA Limits* (2) N/A 10 1,000 1,000 200 700 2,000
* Micrograms per gram dry weight
** Micrograms per gram wet weight
N/A Not Available