Feasibility of Using Feedlot Manure for Biogas Production

Peter Watts and Bernadette McCabe Feedlot Services Australia T/A FSA Consulting and National Centre for Engineering in Agriculture - Meat & Livestock Australia

Type: Research Paper
Knowledge level: Advanced

Farm Table says:

This research article covers the feasibility of using feedlot manure for biogas production and is persistent with US feedlots where “feedlot soil” is harvested.

 

What is the problem?

Beef feedlot manure has a relatively high energy content, similar to other organic waste materials. As such, it offers the potential for energy recovery using thermal processes such as combustion, gasification or pyrolysis and biological processes such as anaerobic digestion.

This task was aimed:

  • To explored the feasibility of anaerobic digestion systems for feedlot manure for optimal biogas production. An initial review of the international research literature on anaerobic digestion of cattle manure and similar waste streams, along with case studies on existing plants was conducted to define the required characteristics of an influent substrate for anaerobic digestion and to identify the current status of energy recovery using anaerobic digestion from feedlot manure, with a particular focus on US practices.

What did the research involve?

2.1 Cattle feedlots
-A cattle feedlot is a facility where beef cattle are housed in open pens and fed a prepared diet until they reach a specified weight (See Photograph 1). Only weaned cattle enter the feedlot and no breeding of cattle occurs at the feedlot.

2.2 Feedlot manure management systems

2.2.1 Manure management overview
-Cattle excrete fresh manure (urine plus faeces) onto the pen surface (known as the feedpad) where it immediately begins to breakdown.

3.1 Feedlot manure sampling

3.1.1 Biomethane potential testing and chemical composition
-Three feedlots in Southeast Queensland with different feed processing technology and pen capacities were selected for this experiment.

3.1.2 Physical characterisation
-Testing of secondary parameters such as turbidity, bulk density and settling velocity of solids was carried out on 3 types of manure (fresh, pen and stockpile manure) from two feedlots (F4 and F5) located in Southeast Queensland.

  1. F4 FM (Freshly cleaned manure from pen)
  2. F4 PSM (Fresh surface pen manure)
  3. F4 SM (Stockpile manure) 4. F5 PSM (Thinly scraped pen manure)
  4. F5 SM (Old stockpile manure)

3.2 Biomethane potential batch tests

3.2.1 Sample Preparation

-A single composite sample was created from the three bags collected from each feedlot pen in the laboratory and immediately processed.

3.2.2 Chemical characterisation TS, VS and MC measurement
-The solid content of the samples was determined according to Standard methods (APHA, 2005). For the determination of total solid (TS), samples of 2 to 4 grams were placed in a ceramic crucible and weighed.

Organic matter composition
-Further analysis of the organic matter content in the form of lignin, fat, volatile fatty acids (VFA), cellulose, hemicellulose, starch and protein was performed by SGS Food and Agriculture (Brisbane, QLD).

.2.3 BMP set up and operation
-A multi-channel analyser – the automatic methane potential test system (AMPTSII, bioprocess control, Sweden) – was used to perform BMP testing of manure samples.

3.2.4 Inoculum (sludge) source
-Inoculum used for every batch test was collected from the anaerobic digestion unit at Pittsworth, QLD wastewater treatment plant. The plant consists of overflow settling tank, anaerobic digester and a trickling filter.

3.2.5 Inoculum: substrate ratios (ISR)
– Manure samples collected from the different feedlots as outlined in Section 6.1 were used as substrates in the biomethane batch tests.

3.2.6 Blanks and controls
-Blank assays containing only sludge (inoculum) and distilled water (1:1) were used to compensate for the amount of biogas produced by the inocula itself (i.e. correct for background methane production from the inoculum.

3.2.7 Preparation of bioreactors and start-up procedure
*3 reactors as blanks containing only sludge (inoculum) + distilled water.

* 3 reactors as positive controls containing only sludge (inoculum) + cellulose in distilled water.
* 3 x 3 reactors with sludge (inoculum) + manure sample (substrate)

3.4 Pre-treatment systems
-Various pre-treatment approaches have been used in the literature, ranging from biological treatment such as incubating with hemicellulose degrading bacterium at 70°C, to thermal (up to 190°C) and thermochemical treatment (with acid or alkaline).

3.4.1 Pre-treatment trial
-Approximately 40 kg of freshly scraped pen manure (FSPM) and pen surface manure (PSM) from Feedlot 3 were collected and sub samples were taken by mixing and quartering the larger manure sample.

What were the key findings?

4.1 Manure characterisation

4.1.1 Chemical composition
-The results from the characterisation analyses of fresh, pad and stockpiled manure samples are displayed.

4.2 Methane yields (BMP)
-The methane production (in L methane produced per kg VS added) obtained from each manure sample at the end of anaerobic digestion.

4.3 Preliminary pre-treatment trials
-The TS and VS contents of the various manure samples collected before treatment.

4.3.1 Effect of stirring on particle sizes
-Pre-treatment was only performed on manure from Feedlot 3 in the preliminary trial. Due to the rainfall event (6 mm) two days before manure sampling, pen surface and freshly scraped pen manure samples had higher moisture content than pen manure samples from Feedlot 1 and 2.

4.3.2 Effect of pretreatment on TS and VS contents
-In Trial 1, both pen surface and freshly scraped pen manure mixes had a higher TS content in the bottom layer than the mid layers, reflecting that complete mixing was not achieved inside the vessel.

4.3.2 Effect of pretreatment on TS and VS contents
-In Trial 1, both pen surface and freshly scraped pen manure mixes had a higher TS content in the bottom layer than the mid layers, reflecting that complete mixing was not achieved inside the vessel.

Physical design parameters of a feedlot biogas plant
-The literature review provided examples of successful manure-based biogas plants operating in both Australia and the USA, mainly for dairy and pig operations.

4.4.1 Manure harvesting
-To maximise methane yield and minimise in-digester sludge accumulation, fresh manure should be skimmed frequently from the pen surface without contamination by soil and gravel.

4.4.2 Manure screening
-Freshly-scraped pen manure should be used as soon as possible to minimise methane potential loss. However, some provisions for short-term storage adjacent to the biogas facility should be made.

4.4.3 Mixing tank
-The influent to the digester should have a solids content of 4-6% total solids with the organic matter broken down into small particles, fully saturated and dissolved.

4.4.4 Digester
-It is proposed that covered anaerobic ponds (CAPs) should be used rather than large in-tank anaerobic digesters.

4.4.5 Biogas handling and use
-The handling and use of biogas produced from CAPs is an existing and proven technology.

4.4.7 Sludge and effluent dewatering
-The effluent leaving the CAP, although digested, still contains some solids. This effluent, as well as the removed sludge, needs to be dewatered.

4.5 Economic assessment of a feedlot biogas system

4.5.1 Proposed biogas system design and indicative cost
-Current feedlot manure management in solid form offers little opportunity for current digester designs.

4.5.2 Fertilizer sales
-The likely impact on fertilizer sales is reported in Table 16.This table compares the income of manure sold as fertilizer with the anticipated income from selling the dried anaerobic digestate (dried sludge) and excess manure.

4.5.3 Emissions Reduction Fund (ERF)
-Currently there is no methodology addressing the disposal of, and emissions related to feedlot manure.

Final comment

The literature focal point is that the economic value of feedlot manure for anaerobic digestion is largely determined by the composition (quality) of the manure. Pen cleaning timing, frequency and method affects the quality of the manure removed.

2015 - Australia - Peter Watts and Bernadette McCabe Feedlot Services Australia T/A FSA Consulting and National Centre for Engineering in Agriculture - Meat & Livestock Australia
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