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Waste characterization and recipe development for composting aquaculture sludge

Composting is an aerobic process where microorganisms are used to decompose organic matter. Typically, decomposed materials lose mass and moisture, accompanied by the transformation and release of nutrients, such as organic carbon, nitrogen, and phosphorus, into water-soluble forms and volatile gases (e.g. carbon dioxide and ammonia). The end product or ‘compost’ is a nutrient-rich, soil-like product that can be used as an amendment or nutrient source in agriculture, home gardens, and lawn management. 

Traditionally, composting uses nitrogen inputs such as food scraps or animal waste/mortalities and carbon inputs such as paper/cardboard waste, woodchips, or wood shavings. Alternatively, composting condensed fish waste solids from recirculating aquaculture systems may offer aquaculture farmers a method to generate value from an otherwise underutilized resource. 

However, the properties of aquaculture waste solids also present some unique challenges. Sludge’s low carbon-to-nitrogen (C/N) ratio and high moisture content may need to be managed using large volumes of dry carbon-rich bulking agents, resulting in elevated operating costs. 

Equipment
Compost can be produced in simple and inexpensive “windrow” piles (Figure 1A) but with reduced control over the process parameters. More expensive and increasingly sophisticated systems, such as rotary drums (Figure 1B) with “smart” control panel integration, may offer precision control of operating conditions via remote operation. More operator control decreases overall compost time and may reduce the amount (and cost) of carbon inputs.

Recipe development
The first step for compost recipe development is to characterize feedstocks, the available aquaculture sludge (i.e. the nitrogen input) and the chosen bulking agent (i.e. the carbon input, generally wood-based). Sludges can be analyzed using standard water and wastewater testing methods. Bulking material will require testing at a facility equipped for soil or compost analysis. 

Generally, feedstocks should be analyzed for pH, moisture, organic matter, and nutrient content. It is also important to note that sludge characteristics may vary daily, and bulking materials may not be homogenous, so several samples should be collected to determine an average for the materials.  

Once the feedstocks have been characterized, the data is used to develop a basic recipe using the different components’ C/N ratio and moisture content. Ideally, the C/N ratio of the mixture (waste substrates + bulking agent) should be within 25-35, while the moisture content should be between 45-65 per cent. With aquaculture sludge having a substantially high moisture content (over 99 per cent without any dewatering unit processes), it may be more economically feasible to aim for a mixture moisture content of 60-65 per cent to minimize bulking agent expenses. 

Dewatering unit processes that help separate and concentrate the waste solids could also reduce the amount of bulking material required. Similarly, the low C/N ratio of the aquaculture sludge (typically around 6-8) may necessitate the requirement of a bulking agent with a relatively high C/N ratio (60-80). 

Controlling the process
While several parameters control the composting process, focusing on the C/N ratio, moisture content, aeration, and pile temperatures is crucial for success. Microorganisms require carbon as an energy source and nitrogen for biomass growth. 

A low C/N ratio attributed to relatively high nitrogen concentrations could lead to odour issues from excess ammonia production. Conversely, a high C/N ratio may result in slower growth rates for the microorganisms, leading to reduced rates of organic matter breakdown and lower pile temperatures for pathogen destruction. 

The moisture content also affects the activity of the microorganisms. Low moisture content could lead to reduced microbial activity due to reduced water availability. High moisture content may lead to reduced access to oxygen, leading to anaerobic conditions, a change in the microbial community, and hydrogen sulfide production, causing odour issues and nitrogen loss. 

There is a higher risk of elevated ammonia and hydrogen sulfide production for aquaculture sludge due to the low C/N ratio and high moisture content of the sludge, especially if an adequate quantity of an appropriate bulking material is not added to the mixture. Improving aeration by turning windrow piles every two to three days or using a blower for an in-vessel composting system may help alleviate anaerobic conditions. 

Pile temperatures in the thermophilic range (excess of 55 C, 131 F) are necessary for pathogen destruction in the finished compost. The EPA (Title 40 CFR Part 503) recommends that the number of days the average pile temperature should stay above the threshold mentioned above for different composting systems. For example, in-vessel composting via rotary drum requires temperatures in the thermophilic range for at least three days to qualify as a Class A biosolid.

Challenges
There can be several challenges associated with composting aquaculture sludge. Some of these issues could be farm-specific, such as sand from biofiltration units in the sludge, which reduces the organic matter content, or the unavailability of dewatering equipment for the sludge. 

Other challenges may arise from the quality and type of the bulking material, which determines aeration demands. Readily available bulking materials with relatively high bulk densities may increase the need for aeration due to compaction and poor porosity in the pile. The increased aeration could create a cascading effect and result in a decrease in pile temperatures. 

However, one of the primary challenges with recipe development for aquaculture sludge is balancing the mixture’s moisture content and C/N ratio. The high moisture content of the sludge generally necessitates the use of a large volume of a carbon-rich bulking agent. In some cases, lowering the moisture content of the sludge to an optimal value by adding bulking material could lead to C/N ratios over the ideal range, which would impact the composting temperatures. 

Co-composting or adding a secondary waste substrate may be necessary to balance these parameters. On aquaculture farms, mortalities or discarded viscera waste after a harvest could act as the secondary substrate for co-composting.

Composting aquaculture sludge could be an effective means of creating additional value from aquaculture sludge. The Conservation Fund Freshwater Institute is currently researching aquaculture sludge composting and co-composting using an in-vessel composting system.