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

Alternative Ingredients and Recipe Development:

After evaluating the potential of a wide range of alternative carbon source ingredients the only one that appeared suitable was horse bedding. Other ingredients, such as soiled silage or rotten hay, that were considered either lacked the ability to absorb the high moisture content of the fish manure or were not available on a consistent basis. Horse manure from three stables was tested (Table 1). Compost recipes for trials were developed with the bedding from the stable that had the lowest moisture content.

Basic concerns in developing suitable recipes containing fish waste intended for composting are the high water content of biosolids (fish feces and feed residue in tanks) and high nitrogen and elevated fat content of brood and smolts. Attempting to dry morts for analysis required an extra day of convection-oven processing and total fat-oil-grease (FOG) analysis revealed 8.3 to 12.8% fat in Brood (full size morts) and smolts (small fish), respectively. While taken alone these values may not appear significant, they represent unusual ingredient at that level for any wastewater or composting process. In a previous hatchery project conducted by these parties (Pietrak at al. 2007), it was established that oil residue in fish is associated with high organic acid content in the biosolids (principally acetic, butyric and lactic acid). This mixture of feces and oil residue ferment anaerobically. This in turn can create a nuisance in the form of odor as an input ingredient and may also result in low pH in the compost, which can retard the aerobic-biological process. On the whole therefore, oil and fatty acid content pose appreciable challenges to proper composting. On the other hand, failure to adequately balance high nitrogen content of fish meat can lead to high pH and copious and annoying release of ammonia – this further can result in the compost not stabilizing properly over a relatively long period of time and impair perception of compost quality.

The most practical recipes were based on combinations of morts and pond sludge along with “carbon sources”. These latter consisted of dry sawdust, peat moss and horse bedding. The respective composition of these is shown below.

In order to construct a proper approach to preparing recipes, the following variables must be taken into account, in decreasing order of significance: 1) moisture content of ingredients; 2) C:N ratio of resulting blend and 3) volume porosity and density. A basic mix of Salmon brood stock with the sawdust to obtain a start C:N ratio of 25 consisted of a 1:2 wet weight mixing of fish to carbon source; this resulted in too dry a mix and therefore water equal to the weight of the fish had to be added; if pond sludge (biosolids) was used for the water source, it would raise the nitrogen (lower the CN) too excessively when used alone.

In preparing compost mixes in the field, the only practically controllable (and observable) parameter is the moisture content, which can be subjectively and accurately checked by the hand squeeze test. Guidance in proper ratios based on tested parameters must come from previous careful lab analyses for each potential ingredient, from which a range of acceptable additions is developed, and used to set ranges for mixing on site. Therefore, sufficient lead-time is important. For example, Woods End analyzed materials and developed the recipes in terms of wet weight, volume and gallons of ingredients, so that after the weighted average analysis in the lab using a spreadsheet had been completed for all possible ingredients, the proposed mix ratio was a bulk volume of as-is ingredients scaled to actual site operations. To further reduce it to man-field scale, small volumes such as 5-40 gallons at a time are workable; this implies that the entire process is labor intensive for the hatchery scale and in-vessel composter targeted in this study.

In the example above, using brood fish plus sawdust, a suitable start blend was as follows: 100 gallons of fish (about two barrels) and 1,000 gallons of sawdust. As it was not practical to measure 1,000 gallons of sawdust, the volume ratios from weight basis was found to be more useful; this required the volume density of the ingredient to have been previously measured. Volume density is the slightly-packed weight per standard unit of volume. In the laboratory we use a specific unit of pressure (4x loose weight) applied to a test sample in a standard-volume container to obtain bulk density that is accurate at the field level. Failure to do this results in underestimating the end volume and therefore over estimating (most likely) the source fish ingredient, which can be very problematic. With these considerations, in the field this can be carried out with standard tote-bins or barrels and buckets. It is helpful if trying to calculate density in the field that a weighing scale is present capable of 0.5kg resolution up to 50 kg. The end result of this exercise for brood and sawdust is that Woods End recommended 9 volumes of as-is sawdust to 1 volume of brood fish.

We also attempted to validate the scientific mixing method by examining individual fish samples. The typical weight of a small mort smolt was found to be 0.8 kg whereas brood stock weight was 3.65 kg. Each full size fish contained 0.1 kg of nitrogen. By calculation, this meant that each fish required 2.5 kg of carbon or approximately 5 kg of added nitrogen-free organic matter in order to basically balance the nitrogen content. (This approach is anticipated to be useful for facilities tossing whole culled fish into compost reactors while they are operating, since each fish must be balanced with some carbon.) It was too difficult to obtain a nitrogen figure for the small fish smolts as the nature for their greater oil content prevented proper use of laboratory equipment; however the weight ratio between the two fish would suffice to estimate mixing proportions, assuming similar relative weights of nitrogen per fish.

When mixing brood fish with sawdust alone, the resulting blend appears to the casual observer to be mostly sawdust with a little fish in it- this we reasoned may seem to be unattractive compost. Even after composting, the appearance of such high sawdust blends is that of moistened sawdust that smelled somewhat like fish (until very mature). Since the goal of the project is not simply to dispose of fish safely, but to create a useful (and attractive) end product (for gardening, as an example), this project also looked at other ingredients that would aid in both the recipe balancing and the final appearance - and composition - of the compost. After a process of examining ingredients potentially available at reasonable cost within the region, it was most practical to obtain horse bedding and peat moss, and to use these in various combinations. These products have reasonable C:N ratios and baled peat was found to be consistently dry and therefore an excellent drying agent - in addition to adding porosity. Horse manure additionally contributes a farm-like aroma offsetting the fish smell, and is reasonably believed to add desirable heterotrophic microbes useful for compost process quality.

Two enhanced recipes were therefore developed; the first consisted of brood fish, sawdust, peat and pond sludge (biosolids), and a full mix consisting of all the listed ingredients, in specific ratios. It was found to be very practical to shoot for a C:N ratio of 25, considered to be a good start condition for any composting, whereas a higher C:N required use of seeming excessive amounts of sawdust or peat.

Enhanced Recipe 1. Brood + Sawdust + Peat Ratios: 1 : 5 : 7 then add in ½ part pond sludge (the sawdust in this mix must be previously moistened with water, as using the pond sludge alone for water would result in too low a C:N and too odorous a mix).

Enhanced Full Recipe: in this recipe we found we could balance all the moisture needs of the initial compost by combining as-is horse manure (60% moisture content) with biosolids to reduce the amount of sawdust and peat. This resulted in a moderately low moisture base mix (estimated 45% water) that would be an excellent starter compost and would allow rapid heating.

The success of early heating must be assured by observing recipes that do not cause over moistening and yet provide adequate nutrient – especially nitrogen- to balance the bedding material. The more reliance I made on sawdust the less likely the early heating will be; however other carbon sources such a horse manure are typically quite moist so that limits the real amount of fish that can be added into the mix. Each facility will have to carefully balance these concerns and objectives.

Gardner Lake Compost Trial #1

The overall goal of this trial was to test the use of horse manure as an alternative ingredient and evaluate the feasibility of integrating composting with the newer wastewater treatment technology. Due to the relatively high moisture content of the horse bedding, we still needed to utilize some sawdust in order deal with the moisture from the fish manure. In addition, manure collected from the belt filter was higher in water content than previous samples taken from settling ponds. This conflicting result, as in theory the belt filter should de-water the manure to some extent, was confirmed by sampling manure off of another belt filter recently installed at a federal fish hatchery. In both cases the water was around 95% of the sample.

The compost made in this trial did not heat as well as previous work, although as seen in the final analyses it tested quite well. As can be seen in the temperature profile shown in Figure 2, the temperature never rose above 50 C. At the workshop hosted at the end of this project, Bill Seekins from Maine's Department of Agriculture provided copies of newly completed state guidelines for composting fish mortalities, developed by the Maine Compost Team. The Maine Compost Team is comprised of compost experts from the Maine Department of Agriculture and the Maine Department of Environmental Protection. These guidelines recommend achieving pile temperatures above 130-150 F or 54-66 C for an extended period of time. This compost batch never achieved a hot enough temperature. It is believed that this was due to the lower available organic content in the compost, a probable result of the higher percent water in the fish manure. It was believed that the higher organic content of the horse bedding might make up for some of this deficiency. These factors illustrate the complexity involved in a seemingly simple process of composting.

In addition to the problem discussed above from the higher moisture levels of manure separated out with belt filters, the system of pumping out a holding tank was both messy and not very accurate. I believe that it could be possible to utilize a smaller hose and less pressure in a system that was custom designed for use with injecting into a compost vessel, however, this would decrease the capacity of the system to still be pumped out into a truck for other means of disposal. In future investigations on the scale of an appropriate system this should be a consideration.

Bingham Compost Trial #2

Compost trial #2 was a better success than the first trial. The goal of this trial was to utilize mortalities as the major feed ingredient. Given the complexities in recipe formulation we opted for a simple mix of saw dust and mortalities in the trial. As in previous work we used multiple additions of ingredients to build up to the final amounts.

The compost heated extremely well as can be seen in the temperature profile for the compost run (Figure3). The compost batch ran successfully achieving 16 continuous days above 54 C. The temperatures that were achieved in the compost complied with the Guidance for Composting Fish Mortalities, developed by the Maine Compost Team and distributed at the compost workshop held on September 19 as part of this grant. The Maine Compost Team is comprised of compost experts from the Maine Department of Agriculture and the Maine Department of Environmental Protection. These guidelines recommend achieving pile temperatures above 130-150 F or 54-66 C for an extended period of time, the same guidelines as exists in the federal EPA 503 rule.

A sample of the compost was taken to Woods End Laboratories for analysis on August 12, 2008. The results of the analysis are reported in Appendix 1. The analysis was favorable with total nitrogen of 1.627%. This level was not as high as we have achieved in the past with fish manure, but certainly equal to or above many commercially available composts. It is believed that the fish to saw dust volume ratio can be lowered and thereby increase the total amount of nitrogen in the compost. It is also possible that due to the high heat, more nitrogen was lost as ammonia.

Compost product evaluation

Two sets of compost runs conducted in the EarthTub® composter at two facilities were considered for compost quality review at Woods End Laboratory. The first run was conducted at Gardner lake and was completed late 2007. The next run at the Bingham hatchery was completed in August of 2008. Full lab reports are attached to this report (appendix 1).

Gardner Lake composting yielded an end-product with a slightly above neutral pH (7.5) and a 2% total-N analysis (dry basis) with a C:N of 16. The respiration test showed a stable product. This would be considered a very marketable product. The salinity was moderately high, in the range that manure compost would be expected to fall (11 dS/m). However, when the compost was used to grow seedlings at a volume rate of 1/3 mix in limed peat (diluted down in order not to exceed a conductivity of 3 dS/m), the plants out-performed the commercial potting mix positive control. Other characteristics of this fish compost were high ammonium content (6,000 ppm) but also elevated nitrate (500 ppm) showing that the protein has moved into soluble N-form and has continued to nitrify, a requirement for aging compost. A “stuck” compost, one that does not mature properly, will have elevated ammonium with little or no nitrate, also associated with a high pH,- neither of these undesired conditions exist in the finished compost.

In contrast to this trial, the finished Earthtub compost conducted at Bingham hatchery had a near-neutral pH (6.5), but was lower in total-N (1.6%) and therefore had a much higher C:N (29). It also showed much lower salinity indicating nutrient loading was much less. This particular compost is not yet a fully finished product if the CN alone is considered – a common factor used to assess compost completion. However, the respiration rate was very low, indicating a possibly stable condition. This points to the compost perhaps having lost N from high heat, and not a deficiency in the process per se.

Conclusions

This laboratory analysis of ingredients and end-product in composting morts and brood stock fish along with biosolids, indicates a significant challenge to balance ingredients carefully in order to obtain a satisfactory composting process. The very high nutrient, especially nitrogen content, along with the high fat content, limit the addition of fish at any one time to a mix, and the high water (and N) content of the feces-biosolids, have a similar influence on preparing an overall initial mix.

Because all these factors are so dependent on companion ingredients, the effort to obtain suitable compost mix ingredients, and to properly pre-test them so the traits are well known, should not be underestimated. A variation in moisture content alone in carbon ingredients could act as an absolute bar on adding fish waste or on its frequency of addition; this in turn could represent a slow-down or a total interruption in a scheduled hatchery plant operation for disposing of fish.

In general, the ingredients chose are those that are easily obtainable; sawdust and horse bedding which can be delivered by the truckload, and peat moss, which can be obtained baled at virtually any commercial nursery and hardware store. Sawdust and peat moss can also be safely stockpiled under cover; stockpiling of horse manure is not possible since the material itself will start to heat and compost alone.

Based on the overall review of the compost ingredients and test traits of the end product, the laboratory recommends careful pre analysis of all ingredients that are not off-the shelf and standardized; that is, while baled peat moss can be inferred from book values or those provided in this report, in contrast, sawdust (due to source and storage conditions) and especially horse manure (due to location and type of farm/hobby operations) are much more variable and present risks to the operation. Since horse manure is highly recommended, it must be repeat tested to understand the variations that could influence quality.

This project used “cold-startup” conditions to make fish compost that is, beginning with a base recipe containing fish and immediately setting the in-vessel unit. A recommended control mechanism for establishing composting process quality is that operations first establish a successful non-fish compost process, preferably employing peat and horse manure. This validates that a satisfactory basic composting condition has been attained (involving positive heating); it also is very helpful to calibrate the in-vessel unit (air flow and rotations). This is not possible if manure is not available for sawdust or peat alone even with moisture added, will not compost. Following this procedure to create a basic starter compost, once the process control is established, the manure content can be slightly reduced and the peat increased, as fish is introduced. The addition of brood stock (entire fish) into existing hot compost has been found in this and earlier projects to be a very successful approach; the use of fish to jump start compost from the start is more challenging if prior conditions of composting without the fish are not known.

This study did not explore as a variable the type of composting technology. The use of very small-scale composting equipment such as the Earthtub, although convenient, may ultimately be impractical, especially if an unsuccessful compost process (i.e. one not-heating) should occur. To unload an in-vessel system or attempt to repair the biological process is relatively difficult and time consuming. The volume limits present another challenge, suggesting that two and not one unit may be more practical. Since the high quality of compost obtainable from fish waste has been established in this study more work is recommended to examine differing scales and technology types of composting for hatcheries. For example, medium scale composting in accessible bunkers using normal on-farm equipment (skid steer loaders) could also be very cost effective and practical. Finally, continual year-round operations must be tested to establish that flow of material and mixing is achievable under seasonal constraints.