February 22, 2011 | General

Cocomposting Of Dairy Processing Sludge (Australia)

BioCycle February 2011, Vol. 52, No. 2, p. 47
Preliminary study indicates an appropriate aeration strategy is necessary due to high energy content of the processing sludge.
K.Wilkinson, J. Issa, P. Cullis, B. Meehan and M. De Blasio

INTENSIVE agricultural industries have been required to comply with higher standards of environmental regulation in many developed countries over the past few decades. In Australia, many dairy factory waste streams are recycled as stock feed, compost or tallow. The rising costs associated with regulatory compliance and waste management are major incentives for industry to regularly reevaluate current practices and seek alternative treatment strategies for their waste streams.
Wastewater from dairy factories in Australia can be treated on-site by anaerobic digestion or discharged to sewer. However, fats are usually flocculated out of the wastewater in order to reduce trade waste charges. Flocculation solids – suspended solids removed from wastewater by coagulation and flocculation – can also be sticky and pasty, release unpleasant odors, and usually appear to resist decomposition when applied to land. On the other hand, fats are organic compounds known to be very suitable for composting.
In this preliminary study, a dairy processing sludge was cocomposted with green waste to identify potential issues for future investigation. This research was conducted as part of the multidisciplinary “Closing the Loop” project, conducted by a consortium including government agencies, research providers and the dairy processing industry.

Research Methods
A dairy processing sludge (“PT sludge”) was obtained from a dairy in Victoria, Australia. It was comprised of flocculated solids recovered from wastewater treatment using a tangential flow separator. It was a “spade-able” solid of about 77 percent moisture content and had fat and protein contents of about 34 percent and 16 percent respectively (Table 1).
Preshredded green waste was supplied by Natural Recovery Systems Pty Ltd. of Dandenong, Victoria. It was typical of Melbourne’s green waste stream when derived from a high proportion of grass clippings. Total N content was about 1.8 percent, with a C/N ratio of about 17 and moisture content of 40 percent (Table 1).
PT sludge, comprising about 25 percent of the mix by weight, was composted with the green waste in four experimental in-vessel reactors for 21 days (Figure 1). The moisture content of the mixes was between 55 and 60 percent. After 24 hours of composting, the aeration settings were reduced in two of the reactors to impose anaerobic conditions (low rate aeration, LR). The other two reactors were operated at a high rate (HR) aeration setting to maintain oxygen levels above about 10 percent.
Gas samples for volatile organic compound (VOC) analysis were collected periodically from the headspace of each reactor. Analysis was performed by gas chromatography-mass spectrometry (GCMS) using thermal trapping and desorption injection.

Preliminary Findings

Temperature and Oxygen Concentration Profile
In the two HR reactors, temperature rose rapidly to over 70°C and was sustained above 45°C for more than 12 days (Figure 2 a,b). From there temperatures gradually declined to below 30°C. In these same reactors, oxygen (O2) concentration dropped initially to below 10 percent then stabilized at around 12 percent during peak temperature production. As the compost started to cool, O2 concentration gradually increased. Temperatures above 45°C were sustained for about two days longer in the LR reactors, though in one of them (LR-1) peak temperatures were noticeably lower. Oxygen concentrations in the LR reactors stayed below 10 percent for about 12 days.
High demand for O2 caused the LR reactors to approach anaerobic conditions only briefly during the first day of switching to the lower aeration setting. The relatively quick recovery of O2 concentration in the LR reactors suggests that the level of aeration used in this study (about 1.5 L/min/kgDM) was probably sufficient for composting PT sludge amended mixes.

Degradation Of Fats

The fat content of all the mixes was reduced from above five percent to below two percent after 21 days composting. The rate of fat degradation did not appear to be affected by the aeration strategy.
Small to moderate amounts of fat in a mix can enhance the composting process probably because of its high energy content. The proportion of PT sludge, or other high fat wastes, that could be incorporated in a mixture is likely to be limited by the porosity of the final mix and the ability of the mixing equipment to mix evenly and break up clumps of solids.
Porosity can also be affected by varying the type and particle size of the bulking agent when fats are incorporated. For example, Lemus et al. (see “Bench-scale study of the biodegradation of grease trap sludge with yard trimmings or synthetic food waste via composting,” 2004, J. Environ. Eng. Sci.) showed that the rate of degradation of fats during the high rate phase of composting varied between 10 to 51 percent depending on the mixture and bulking agent used. Poor degradation of fat occurred when it was mixed and composted with dry dog food. High fat degradation occurred when mixed with green waste.

VOC Analysis

A large range of VOCs was detected at low levels; the more abundant of those are listed in Table 2. Many of these VOCs are commonly associated with green waste and municipal solid waste composting facilities. Of the known malodorous compounds, only 2-butanethiol – a mercaptan that has the repulsive characteristic of skunk odor – was detected among the most abundant VOCs. Mercaptans are formed from the aerobic and anaerobic decomposition of sulphur containing amino acids, but production under anaerobic conditions is greater. Mercaptans can then be oxidized to dimethyl sulfide and dimethyl disulfide, two compounds with very low odor thresholds.
In this experiment, the evolution of total VOCs was found to reach a maximum after approximately 36 hours and then rapidly decreased to lower levels. A smaller increase in the lighter hydrocarbons was also observed after about 300 hours.

This study has shown that dairy processing sludge can be successfully cocomposted with green waste. PT sludge was included in the mix at between 10-15% dry solids — at this rate there appeared to be no detrimental effects. High proportions of PT sludge, or other high-fat sludges, in a mix could reduce porosity and inhibit the composting process. The high energy content of dairy processing sludges will result in generation of high temperatures during composting, which need to be managed by an aeration strategy that minimizes losses in nutrient value and odor emissions. Further work should investigate different approaches to composting dairy processing sludges (e.g. in-vessel and windrow) with respect to losses of NH3 and other odorous compounds.

Kevin Wilkinson is with the Future Farming Systems Research Division, Department of Primary Industries, Parkville, Australia. Jason Issa, Peter Cullis, Barry Meehan and Maico De Blasio are in the School of Applied Sciences, RMIT University, Melbourne, Australia. Other research reports, including best management practices for composting dairy processing sludge, can be found at

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