Evaluation of various recipes and ingredients for composting aquaculture fish waste to attain a stable, high-nitrogen end product

View the project final report

Commodities

• Animals: fish

Practices

• Animal Production: aquaculture
• Crop Production: nutrient management
• Soil Management: composting

Land-based fish farms such as hatcheries are often cited as being more environmentally sustainable since they allow for any discharged water to be treated by modern sewage-treatment technology. Indeed, land-based recirculation systems such as hatcheries can use various technologies to remove unwanted solids and nutrients from wastewater streams. Current technology for treating waste can vary greatly from hatchery to hatchery. In order to impose minimum standards, the Maine Department of Environmental Protection (MEDEP) has imposed on discharge permits that all hatcheries--state, federal, and commercial--install, if not currently present, 60-micron micro-screen filtration of effluent. This standard was established after an independent review of 11 state and federal hatcheries in Maine. The MEDEP also requires the maximum amount of total suspended solids be limited to a monthly average of 6 mg of solids per liter of discharge water with a daily maximum value of 10 mg per liter (MEDEP hatchery discharge permits for Craig Brook National Fish Hatchery, Permit # ME0002437). These standards will help to protect Maine’s natural waterways. Unfortunately, current treatment technologies create problems of their own, since they do not actually solve any problems but rather collect and concentrate to various degrees the excess feed and feces that are the source of the nutrient pollution. By limiting the exposure of discharge water to the sources of nutrients, hatchery managers can reduce the amount of polluting nutrients in discharge water. These solutions still leave the cause of the problem-- the excess feed and feces--in a now more concentrated form or sludge that hatchery managers must still dispose of in an environmentally benign manner. As hatcheries come into compliance with these new standards, they will increasingly have to dispose of larger and larger amounts of more concentrated sludge. This sludge is collected on site in either a pond or holding tank after undergoing the various filtration and concentration treatments. Currently, two primary disposal methods exist for this sludge. The first and most common method is to find farmers in the area who are willing to use the sludge on their fields as manure. Previous published studies have shown that the chemical composition of fish fecal materials is similar to other animal manure. This disposal method is preferred, as it is relatively inexpensive and does provide for the reuse of this waste material. However, this option is becoming more and more limited as farmers are increasingly regulated on the amount of nutrients allowed to run off their farms. Due to these new restrictions, farmers are struggling to find acceptable ways to deal with the manure produced by their own animals and are often not willing to try to deal somebody else’s as well. This means that operators increasingly have to use the second disposal option, which is to call in septic or sewer disposal companies who will haul off the waste and dispose of it for a fee. This can result in large expenses for hatchery operators, which increases the cost of production. The Maine Aquaculture Association (MAA) believes that new options for disposal can be found that allow for the reuse of these wastes at a lower cost then paying for disposal. One option we have been developing is a method for composting the sludge in order to create a useable by-product from this waste. To this end, we are collaborating with Woods End Laboratories, a Maine-based company that specializes in compost formulations and waste reuse. In previous research funded by Maine Technology Institute (MTI) and the Maine Aquaculture Innovation Center (MAIC), we have developed a successful compost recipe and conducted a demonstration run with existing in-vessel composting equipment at the partner hatchery. However, this experience has led the hatchery to raise a series of issues that we wish to address in this work. Addressing these issues will help demonstrate to the company the feasibility of composting their sludge. Their issues are: Cost and availability of ingredients other than sludge Development of recipes to compost mortalities from culling and spawning Practicality of integrating the existing demonstration system with existing hatchery treatment systems The time the product needs to be cured before it is marketable

We propose to use the experience gained by the Maine Aquaculture Association, Woods End Laboratories and True North Salmon US in past composting experiments to develop new compost formulations that will address the issues raised by the farmer in our past trials. In particular, we plan to assess the quality and cost of ingredients that are available near each of the three hatcheries operated by Cooke Aquaculture. The goal of this work will be to clearly identify the controlling factors in order to facilitate successful implementation in the hatcheries.

We will then develop and test two new recipes. One will be designed to incorporate a wider variety of local ingredients at the lowest cost combination, while the second recipe will focus on composting mortalities. Local ingredients high in carbon suitable for stabilizing the nitrogen content include (but are not necessarily restricted to) bark-fines from wood processing mills, waste straw from local grain operations, peat moss, and paper pulp from Maine plants previously found to contain no dioxin residue. Following the completion of each run in the compost vessel the compost will be left to cure under compost fabric for up to a year.

Previous collaborative effort has shown that, with the proper recipe, control of compost pH, and curing, a very high nitrogen content compost may result. In the previous study funded by Maine Technology Institute and MAIC, Woods End found in-vessel composting of the fish residue resulted in stable compost with 3.1% N (dry basis), considerably higher than outdoor compost windrowing of fish wastes from processing plants (to date all known attempts to windrow compost hatchery wastes have failed). Periodic testing of the curing compost will help to establish the minimum time needed to cure a product before it can be marketed.

In order to demonstrate the practicality of integrating the proposed test system into existing hatchery treatment systems, we will conduct the compost trial focusing on local ingredients at the Gardner Lake hatchery. This will demonstrate the feasibility of integrating the in-vessel compost system into existing modern hatchery treatment systems. This hatchery has a belt filter and sludge holding tank that can be pumped out. The Bingham hatchery, along with the state and federal hatcheries, have planned treatment-facility upgrades in the next several years. At Bingham, these will likely involve adding a belt filter and sludge storage tank to treat the backwash water from the existing 60-micron drum filters. At the state and federal hatcheries it involves the construction of a new waste treatment facility; in all cases, systems similar to the one at Gardner Lake are likely to be the most common design. Therefore it makes sense to examine the potential issues for integrating the compost process at the Gardner Lake hatchery.

The second trial focusing on composting mortalities will be conducted at the Bingham hatchery where the spawning fish are raised. Bingham was also the hatchery where the previous demonstration experiment took place. Their existing treatment system includes a sludge settling pond instead of a belt filter and storage tank, and the plan is to replace the existing settling pond with a system that will likely be similar to Gardner Lake.