Informative Articles: 
Using fate of Incoming Solids to Measure Manure Digester Performance

USING FATE OF INCOMING SOLIDS TO MEASURE MANURE DIGESTER PERFORMANCE

R.P. Mattocks and Mark A. Moser*

ABSTRACT

Anaerobic digestion is an effective unit process available for animal producers to treat manure. However, anaerobic digesters have met with limited acceptance in the United States.  Animal producers sense a risk that the system, once installed, will perform differently than claimed by the designer.   Performance data is presented for two mesophilic plugflow dairy manure digesters which can be used to benchmark performance values of other mesophilic digesters processing dairy manure. Standard laboratory tests were performed on raw manure, effluent, grit and debris, biogas, screened solids and the nutrient rich separated liquid.  Mass, solids and nutrients entering the digester are observed in the various end products.  Using the methodology, digester designers will be able to demonstrate to investors, how expected outputs of their digestion systems compares to the benchmark values. Investors will be able to make business and economic decisions based on reliable performance numbers.

INTRODUCTION

As American animal populations concentrate into smaller numbers of larger complexes, the environmental impact of the facilities are being more closely watched. Odor, ground and surface water degradation are the principal concerns (U.S. Department of Agriculture, U.S. Environmental Protection Agency, 1998). Of the multitude of manure treatment systems available to reduce these concerns, only methane digesters provide a potential for direct economic return as well as treatment.

Environmental Benefits of Anaerobic Digestion

Environmental benefits of manure digestion have become at least as important as the economics (Moser, et al., 1998a). A stable digestion system will produce an effluent, when compared to raw manure, with significantly less odor, pathogens, weed seeds, and particulates (Moser, et al., 1998b). Nutrients are premineralized to forms more readily utilizable by plants (Marchaim, 1992). Fly potential is also greatly reduced. Few biologically reactive organics remain. Lagoon storage treatment systems designed on organic waste strengths will be smaller with digester effluent then untreated raw manure.

Predicting Digestion Systems Perfomance

Investors seek high probability installed systems will be successful. Adequacy of design is the

single greatest reason for past system failures. Lusk (1998), wrote that when selecting the “best” qualified contractor to design or install an anaerobic digester system, an investor should rarely consider a firm without significant practical field experience.

For a given type, quantity and characteristic of waste, a qualified designer will demonstrate, with some degree of accuracy, the anticipated outputs from a system:

1.    Quantity of gas available for use as an energy source,

2.    Nutrients to be land applied,

3.    Volume of treated material to be transported,

4.    Quantity and characteristics of the recovered solids.

One mechanism to determine performance is to calculate a mass balance for the system. According to Metcalf and Eddy (Metcalf and Eddy, 1991) system performance data is preferred to using book table values. “To prepare a mass balance, it is necessary to have information on the operational performance and efficiency of the various unit operations and processes used in the processing of the waste”. The experienced consultant will have “real world” performance data upon which to base design.

Mass Balance Concept

The fundamental mass balance concept is to reconcile the sum of each constituent entering a digestion system (in what ever form) with the sum of the same constituent leaving the system (in what ever form). All that enters the system must be accounted with what exits the system.

Figure 1 illustrates that in a digestion system there are a number of sources of each constituent: various manures and other organic wastes. Similarly, constituents will exit the system in a variety of forms: biogas, grit, debris, solids, processed liquid.

Figure 2. Basic Mass Balance Concept for Any One Constituent

OBJECTIVE

This paper demonstrates the use of a simple, non-academic methodology to create a system mass balance which can be used to benchmark the performance efficiencies of plug flow mesophilic dairy manure digester.

METHODOLOGY

Terms are defined, followed by the steps used to determine the system mass balance.

Terms Used

Certain terms used throughout the process, are defined:

1. Constituents

Manure and other organic wastes contain total solids, volatile solids, nitrogen, phosphorous, potassium as well as other nutrients, each a constituent of that waste.

2. Hydraulic retention time

The average duration a quantity of treated waste resides in a reactor is referred to as the hydraulic retention time and may be determined by the volume of the digester divided by the quantity of influent fed the digester daily.

3. Influent

Manure and other organic wastes may be influent into the digester for processing.

4. Output products

Biogas, fiber, grit and filtered liquid are output products exiting a digestion system.

Proceedure

The complete system is diagramed. All physical characteristics and activities are noted. This may include manure or waste collection areas, mixing tanks, grit collection equipment, the digester, gas collection and metering, digester effluent tanks, solids separation devices, and storage and ultimate disposal of the liquid leaving the system.

An inventory is made and a diagram made for all materials entering and exiting the studied system. incoming and outgoing flows. This will include any manures or waste products loaded into the system. This will also include grit collected, gas recovered, solids separated and liquid passing out of the separation phase.

The system is monitored to determine when it is at steady state, i.e. when for the period of an entire hydraulic retention time:

[1] of the average value of the period

The system is operated a minimum of three retention times before starting the data collection.

After steady state has been maintained for three successive retention times, during a successive fourth retention time samples and data are collected. For a minimum of 4 days during the fourth retention time: 

1.     Operate the system routinely: feeding the digester, recovering gas, grit and debris, and operating the solids separation system,

2.     Record quantities of influent and output products,

3.     Record densities of the influent, effluent, recovered solids, processed final liquid, grit and debris,

4.     Collect 6 one liter samples each of the digester influent, effluent, recovered fiber, filtered liquid, grit and debris,

5.     The samples should be collected at approximately equidistance between starting and ending of an operation,

6.     Composite each of these into one liter samples, refrigerate,

7.     After the fourth day of sample collection, send all composited sanples to a laboratory certified to employ standard ASTM, AOAC, and ISE laboratory analytical procedures; keep samples refrigerated during transport to the laboratory,

8.     Collect in an appropriate gas tight vessel, one liter of biogas, when the digester gas production is at the lowest production per minute,

9.     Transport the gas sample, within two hours, to a laboratory certified to determine methane, carbon dioxide and hydrogen sulfide concentrations,

Calculation

Laboratory results are analyzed for averages and ranges of each constituent in the incoming feedstock, and the average quantity of constituent of each of the output products.

A determination is made of the averaged quantity of each constituent in the influent, grit and debris, biogas, separated liquid and processed liquid. Laboratory results, volumes and densities are used to make the calculations.

For each constituent entering the system, a percentage is determined as to where that constituent left the system. An example is provided in Table 1. of the possible fate of any constituent loaded into a digester.

Table 1. Example of a Possible Constituent Balance

RESULTS

Performance was observed and samples were collected at two RCM (Resource Conservation Management, Inc., Berkeley, CA) mesophilic plug flow digesters designed for manures from  1000 completely confined cows. Hydraulic retention times were 22 and 20 days. Dry matter intakes were similar though the principle roughage at one site was grass silage whereas corn silage was used at the other. Both dairies pumped digested effluent through FAN separators to recover fiber.  One digestion systems A and B had functioned 18 and 15 months respectively.

Table 2. Physical and Chemical Characterisitics of the Digester Systems Flows

Samples were collected over several days, many hydraulic retention times after the systems were stable. Laboratories, familiar with manure, analyzed the samples. Table 2 reports laboratory and field results.  All values are in percent “as delivered”, except: volumes were related to the incoming manure “feed” volume, biogas densities are reported as kg/m3, sample densities are in kg/m3, and certain nutrients were reported on a ppm basis by the laboratory.

Computation were made to determine the quantity of each constituent in raw digester feed manure, effluent, fiber, filtrate, and to a lesser degree the biogas. The absolute quantity of these various constituents was then related to the same constituents in the incoming manure feed.

Table 3 reports the attempt to balance total constituent mass in with total constituent mass out.  Columns 1 and 2 compare the sum of biogas, fiber and filtrate constituents to the same constituents in the incoming manure. Many of the sum total constituents are within +10% of the expected balance, notably: moisture, total solids volatile solids, nitrogen, potassium, and sodium.

Columns 5 and 6 compare constituents in the effluent of the digester to the incoming manure. Moisture in the effluent is +5% of the influent. Effluent total solids and volatile solids are <50% of the incoming.

     Table 3. Distribution of Digester Influent Constituents

Constituent values are similar for each dairy. Table 4 summarizes results for the dairy systems.

DISCUSSION

The mass balance methodology applied to two dairy manure digestion systems provided readily apparent output trends. Animal production facilities with similar species, breeds, weights, rations and manure management can expect similar results. For the concentration of manure at these two dairies, the output volumes of biogas, fiber and filtrate were about 400%, 28%, and 85% that of the incoming manure volume, respectively. Total solids entering the system, exited (approximately) in the form of biogas (30%), fiber (30%), and filtrate (35%). Fiber contains 17%, 23%, and 13%, of the N, P, and K entering the system, respectively. Filtrate contains 80%, 65%, and 85%, of the N, P, and K entering the system, respectively.

It should be noted that the digestion process alone reduced nutrient levels little. The quantity of material settling in the digester is small. Disposal of nutrients that may settle in the digester will take place later.  The quantities of nutrients entering the digester were often very close to the levels observed in the effluent. However, when digester effluent is passed through another unit process (screen, separator, settling basin, etc), nutrients will be found in both the separated material and the processed liquid. Hence, liquid exiting a manure treatment system will only have less nutrients if there is a solids separation component.

CONCLUSIONS

Investors should expect manure designers to compare outputs from their manure digestion systems to these examples. This paper presents results from two dairy manure mesophilic plug flow digestion systems which should be used as benchmark values against which other similar system should compare.

REFERENCES

Moser, M.A., R. P. Mattocks, S. Gettier, K. Roos, 1998a. Benefits, Costs and Operating Experience at Seven New Agricultural Anaerobic Digesters. ___________.

Moser, M.A., R.P.Mattocks, S. Gettier, K. Roos. 1998b. Keeping Neighbors Happy-Reducing Odor While Making Biogas. Proc. Animal Production Systems and the Environment. Multistate Consortium on Animal Waste, Des Moines, IA.

Marchaim, U. 1992. Biogas Processes for Sustainable Development. Food and Agriculture Service Bulletin 95. FAO, Rome.

 U.S. Department of Agriculture, U.S. Environmental Protection Agency, 1998,

 Unified National Strategy for Animal Feeding Operations, September 11, 1998.

Lusk, P. 1998. Methane Recovery from Animal Manures: A Current Opportunities Casebook. 3rd Edition. NREL/SR-25145. Golden, CO: National Renewable Energy Laboratory. Work performed by Resource Development Associates, Washington, DC.

Metcalf and Eddy, Inc. 1991.Wastewater Engineering Treatment, disposal and Reuse. Revised: Geo. Tchobanoglous and F. L. Burton. McGraw-Hill. New York.

* R. P. Mattocks, Principal, ENVIRONOMICS, P O Box 371, Riverdale, New York, 10471, U.S.A., M.A. Moser, President, R. C. M. Inc., P.O. Box 4715 Berkeley, CA, 94704, U.S.A.

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