Passively Aerated Bunker System for Composting Horse Manure: A Demonstration Project

Introduction

On farm composting of livestock manure has been shown to be an effective means for manure management where a valuable end product is produced from the waste. Over the years there have been many different types of systems developed for large composting systems. Horse stables across the province of Alberta are now finding that manure disposal is becoming an issue and are faced with inadequate land mass to properly dispose of the manure. Stables with 30 - 40 horses do not generate sufficient volumes of manure to merit expensive specialized composting equipment. Windrow turners prices can range from $200,000 to 400,000. The economics of composting can often be a determining factor of whether composting is suitable manure management.

Manure management is an integral component of the equine industry. Currently, it is common to have on site storage of manure for land application. However, this can often lead to odours or other hygiene issues on the farm. It is important for horse stables to be able to adequately manage manure in a way that the health and welfare of the horses or workers are not compromised. Furthermore, there are means to handle the manure effectively to transform the waste into a resource. Composting is often viewed as a technologically simple way of managing manure. There are many advantages of composting manure versus stockpiling. For example, after the manure is composted there is a significant reduction of odour and minimum leachate production. Therefore, composted manure can be stored without becoming an environmental hazard for longer periods of time. The high heat destroys weed seeds and pathogens during composting reducing the potential for pathogens from entering the ground water when the compost is spread on the land. The greatest advantage of using composting for manure management is the creation of a usable and sellable product. Some industrious farmers have been able to produce very high quality manure composts and have been able to turn their waste into a profitable resource.

This paper is the summary of a demonstration project sponsored by the Alberta Agriculture, Food and Rural Department, Horse Division. This project was identified as a method to demonstrate on-farm composting technology for mid size stables. In addition to monitoring the composting performance of the horse manure, stable management practices were also monitored. The amount of bedding inputs were collected and compared to manure outputs. This information was used to identify how stable management might influence the composting program. There may be distinct materials handling difference between composting straw bedded manure versus sawdust. Majority of the manure that is disposed for composting is comprised of bedding material. This may have a critical influence on the composting success. When composting is considered as an integral aspect of horse husbandry, there may be some management changes that need to occur to ensure success in both areas.

Methods and Materials

This demonstration research project was conducted at the Olds College indoor arena which houses approximately 40 horses. Manure from 24 box stalls were collected and used as the main feedstock for composting. A partially enclosed compost bunker system comprised of four passively aerated static piles was constructed to process a year round flow of manure, from the initial raw material stage to a finished product. This system was selected because of the specific advantages over windrow composting for the volumes of manure that are generated from horse stables with 30 - 40 horses. The only piece of machinery required for this system was a front end loader. The composting system was constructed from treated lumber and lined with plastic panels on the inner surface. A sheet metal roof with built-in sprinklers was constructed for controlling moisture in the pile. Perforated 6" PVC pipes were placed in the center of the compost piles allowing passive movement of air to the center of the piles.

The composting performance evaluation was divided into two phases. During the first phase the horses were bedded on sawdust and the stable cleanings were collected daily for composting over a four month period. During the second phase the horses were bedded on straw. This allowed for the evaluation of the composting system on the two most common types of bedding materials. The size of the bunker system was calculated based on the capacity required to treat manure and bedding from 25 horses. Data was collected for both phases of the study to determine the amount of bedding inputs in each stall and the amount of cleanings that were taken to the composting system.

Passively Aerated Composting Bunker at the Olds College Arena.

The composting system had four bunkers and the materials rotated through the bunkers based on the fill time of bunker 1. The students would collect the manure in wheel barrels and take the manure from the stalls to bunker 1. Bunker 1 was used as the primary bunker where fresh manure was deposited daily. Once bunker 1 was filled, the contents would be removed and transferred to bunker 2 by using a front end loader. Once again fresh manure would be placed into bunker 1. Then as bunker 1 is filled all bunkers are rotated again. The following schedule describes to flow of materials through the bunker system (4 week cycle).

Composting schedule:

For each phase of the project the composting performance was monitored by measuring temperature and oxygen in each bunker on a daily basis. Temperature data was collected by using 16ft long stainless steel probes inserted at 5 different locations in each bunker. Oxygen was measured using a Bacarrach oxygen meter with a 5 ft probe. Moisture content and pH were taken from composite samples comprised of 5 sub-samples from each bunker. Moisture was determined gravimetrically by drying in a 60oC oven and pH was measured using a glass pH electrode.

Composite samples were collected for microbial analysis. Fecal coliforms were enumerated using Lauryl Sulphate broth (MPN) and Salmonella sp. was enumerated using Salmonella/Shigella specific media (SS Agar).

Results and Conclusion

The results from this demonstration trial showed that it took four weeks to fill the primary bunker and therefore the residence time for the material to compost in the system was 16 weeks. This was true for both sawdust and straw bedding. Table 1 shows the total amount of bedding input and manure output for each stall over 90 days. The numbers show that all the daily inputs of bedding are taken to compost with the addition of manure. On average 0.08m3 of sawdust bedding is used per day per stall. This translates to 40% of a heavy duty wheel barrel (0.2m3) and 0.1m3 of sawdust/manure to compost on a daily basis. Majority of the manure transferred to the composting system consists of mainly bedding. For this reason the starting C:N ratio of the sawdust manure was 81:1. This is an high initial value for composting systems where the norm is around 30:1. For this reason, urea (46-0-0) has to be added to the sawdust bedded manure for effective composting. Data for straw bedding input and output was not available at the time of this report.

Table 1: Sawdust bedding input and manure output for a total period of 90 days.
Stall #	Total Input (m3)	Total Output (m3)
0	1.4	2.4
4	5.2	5.1
5	7.3	11.9
6	8.9	11.4
7	6.0	8.5
8	9.5	13.4
9	9.9	13.2
10	7.2	10.6
11	9.7	12.6
12	6.8	8.7
13	9.8	13.2
14	7.5	10.0
15	8.4	12.2
16	8.9	11.5
17	8.5	11.6
18	7.0	11.9
19	9.3	12.6
26	7.5	10.9
27	8.9	10.6
28	7.1	9.7
35	8.8	12.0
36	8.3	12.3
37	5.8	7.4
46	0.7	1.0
Total	178.2	244.4

The temperature recordings taken from the sawdust based manure was mostly within 45 - 70oC throughout the system (Figure 1). A similar pattern was observed for the straw based manure (Figure 2). The first batch was started in January when the ambient air temperature was close to -15 to -20oC. However, temperatures started to rise within a few days upon addition of urea and insertion of the aeration pipes. All three batches composted within the thermophilic temperature range (40 - 65oC) for over six months. Sustaining thermophilic temperatures in the composting materials is required for destruction of pathogenic microorganisms such as fecal coliforms and Salmonella sp. Furthermore, the high temperatures also indicate vigorous microbial activity which will result in a shorter processing time.

Figure 1 Temperature profile of the sawdust based maure.

Figure 2 Temperature profile of the straw based manure.

High composting temperatures are sustained by adequate oxygen levels. Aerobic microorganisms require oxygen to breakdown organic matter into compost. The objective is to maintain oxygen levels within the composting materials above 10%. Below this level the compost will become anaerobic and odours will be generated. Figures 3 and 4 show the oxygen levels in the sawdust and straw based manure. High oxygen levels indicate that the passive aeration system was effective in supplying oxygen.

Figure 3 Oxygen concentration for the sawdust based maures. Arrows indicate the insertion of pipes in the bunkers.

Figure 4 Oxygen concentration for the straw based maure.

The sawdust based manure had adequate moisture levels and water wad added infrequently. However, moisture was a limiting factor for the straw based manure. Water addition was necessary on a daily basis for the fresh materials entering bunker 1. Moisture content of the fresh straw/manure was close to 35% (Figure 5). Therefore, large amounts of water was required to increase the moisture content to 50 -60%. The sprinklers were used for the straw/manure, however due to whether challenges it was not always possible to add water to the manure. The third batch of sawdust manure did not compost very well due to the inability to use the sprinklers during the cold spells (Figure 2).

Figure 5 Moisture content of the first 2 batches of straw based manure. Red triangle indicates the moisture content prior to water addition.

In general, this passively aerated composting system was found to be an economical composting system that can produce high quality compost with minimal labor requirements. Sawdust based manure proved to be less complicated than straw. The sawdust seemed to have better ability to retain moisture from the urine and generally took a shorter time to reach maturity (data not presented). The lack of moisture in the straw and it's structure resulted in a longer time required to achieve a mature compost. This system was designed for a four week cycle. However, 16 weeks was not sufficient time for the straw manure to reach maturity. Therefore, future designs for these bunker systems for straw based manure will need to take into account the longer residence time required. This can be achieved by increasing the size of the primary bunker or by having two primary bunkers.

Stable management is an important aspect for the sawdust manure compost. High levels of bedding entering the composting system will result in an imbalance in carbon content for composting. Typically, manure will increase the nitrogen content of the bedding material. Therefore, if the waste consists of more manure, there may not be need to add nitrogen in the form of chemical fertilizers. Whilst it is important to maintain hygienic conditions in the stalls there may be some benefit to allow the bedding to become more soiled prior to discarding to the composting system. There should be sufficient experience from the stable managers to determine when bedding materials are causing noxious odours, or the threat of aerosols (microorganisms and dust) and hence the need to remove the bedding.

On farm composting systems need to be economical and efficient to gain popularity. This demonstration project has enabled for preliminary evaluation of such a system. The data collected from this trial will allow for more knowledgeable predictions and trouble shooting protocol for technology transfer to the end user.

About the Author

Dr. Donna Chaw was the Lead Research Scientist at the Composting Technology Centre at Olds College. She was actively involved with curriculum development for the Commercial Composting Diploma program as well as conducting numerous research projects on composting. Dr. Chaw received her doctorate degree from the University of Hong Kong in the area of hog manure management. She currently works for Alberta Environment as Waste Policy Advisor with Waste and Pesticide Environment in Edmonton.

This information was presented at, and appears in the Proceedings of, the 2002 Alberta Horse Breeders and Owners Conference.

Donna Chaw, Ph.D.
Composting Technology Centre
Olds College Centre for Innovation