The Cultured Abalone aquafarm produces red abalone (Haliotis rufescens) in a land-based system of tanks provided with pumped seawater and using fresh marine kelp (Macrocystis pyrifera) as forage. Farm effluent, monitored by California Regional Water Quality Control Board under a General NPDES Permit for Aquaculture, consists of seawater and digested kelp forage. The purpose of this study was to characterize abalone production effluent in order to determine the feasibility of effluent reclamation for reuse, as an alternative to expanding farm pumping capacity.
Concentrated abalone effluent solids were found to be slow-settling and fine relative to other aquaculture effluents. Additionally, analyzed dry matter solids were relatively low in nitrogen and high in ash. A sedimentation type basin for settling abalone effluent solids was defined for solids removal based on data for settling velocity of effluent particles. Such a basin would require 1 square foot for each 1 gallon per minute of flow in order to capture 84.51% of abalone effluent settleable solids.
Results from this study were presented to attendees of the Aquaculture America conference, February 13-16 2005, in Las Vegas, NV.
Viewing waste water as a valuable resource available for serial reuse is in line with sustainable agriculture goals and the development of secondary products using farm outputs. The first step in development of an integrated production strategy is characterization of outputs, to determine the cost of reclamation and to measure the opportunity a reclamation program represents.
The objective of the study was to initiate a long-term feasibility study of integrated aquaculture production by obtaining data on settling velocity and particle size of suspended solids associated with abalone effluent. These data were used to develop a treatment strategy for abalone effluent suspended solids removal, and subsequent reuse of seawater resources.
First efforts to sample effluent did not find any settleable solids. Sampling efforts were repeated during several phases of farm production, including feeding, high density of algal forage, and low density of algal forage. Settleable solids were only evident in the effluent stream during periods of harvesting, when culture units were drained of water prior to removal of abalone. Any solids present in the effluent during harvest resulted from a surge of water, re-suspending solids that had settled in the drain.
In order to obtain a sufficient amount of effluent solids to analyze, we simulated a large-scale harvesting operation in which a new tank was drained every 2 minutes for a 2-hour period. During this period, at 15 minute intervals, 5-gallon samples were taken from the effluent discharge. These 5-gallon samples were allowed to settle for 24 hours, after which the settled layer was carefully and completely siphoned into a 1-liter sampling jar. Sampling jars were mixed and used for subsequent analyses.
The characteristics of concentrated effluent measured were particle settling velocity and particle size distribution. Settleable solids dry matter was subjected to analysis for total ammonia nitrogen, nitrate nitrogen, nitrite nitrogen, kjeldahl nitrogen, and fixed (ash) and volatile solids. In trials for particle settling velocity and particle size distribution, suspended solids were measured using Standard Methods for the Examination of Water and Wastewater (American Public Health Association, 1995), and particle settling velocity methods were taken from Wong and Piedrahita (2000). Laboratory analysis of dry matter was performed by Creek Environmental Laboratories, San Luis Obispo, CA.
Samples of concentrated effluent solids were introduced into a settling column with a conical bottom, as described in Wong and Piedrahita (2000). Samples were withdrawn from the bottom of the column at 60, 120, 240, 480, 960, and 3600 seconds from the time the sample was introduced. The time elapsed and the height of the settling column was used to calculate the settling velocity, in centimeters per second, of the particles present in each sample. Each sample was filtered, dried, and weighed, and then calculated as a proportion of the total settleable solids in the concentrate. The particle settling velocity data is shown in Table 1.
Table 1. Particle settling velocity data from concentrated abalone effluent. (Data available in written final report.)
The fastest settling solids in the sample, those that settled in the water column at a velocity of 2.10 cm/s, made up only 1.3% of the total settleable solids load in the effluent sample. The right-hand column of Table 1 shows the cumulative fraction of the solids load; after 2 minutes 6.66% of solids had settled, after 4 minutes 33.72% of solids had settled, and after 8 minutes 65.65% of solids had settled. The slowest settling solids in the sample, 15.49% of the sample by mass, took between 16 minutes and 1 hour to settle. Abalone effluent solids settle relatively slowly; by comparison, from trout farm effluent 74% of solids settled at a velocity of 1.03 cm/s or faster (Wong and Piedrahita, 2000), whereas only 5.36% of abalone effluent solids settled at that velocity.
Samples of concentrated effluent were also screened through a series of decreasing mesh sizes, filtered, dried, and weighed in order to determine relative proportion of particle sizes. Samples were screened using 800 micron, 400 micron, 300 micron, and 225 micron mesh sizes. The proportion of solids in each size range is presented in Table 2.
Table 2. Particle size data from concentrated abalone effluent. (Available in written report.)
Fully 66.4% of solids from effluent samples were found to be smaller than 225 microns in size. Settling is generally accepted as an effective method in the removal of effluent solids greater than 100 microns in size and impractical for smaller particles, particularly solids smaller than 30 microns. Mesh smaller than 225 microns was not used in this trial, but the relative slow settling velocities found for abalone effluent solids is in line with a high proportion of small sized particles.
Using the settling velocity data, the dimensions of a settling pond, or sedimentation basin, were determined for removal of solids from abalone effluent. The settling basin is a simple solids removal device, essentially a long rectangular basin where effluent enters at one end and exits the other. Solids that settle before reaching the exit of the basin are removed from flow. A settling pond large enough to accommodate 1000 gallons per minute of abalone effluent would need to be 25 square meters in order to capture 65% of solids, and 51 square meters to capture 84% of solids. By contrast, only 6 square meters would be required to capture 73% of trout effluent solids. Clearly sedimentation may only be an appropriate approach to solids removal of abalone effluent when space is abundant.
One of the principal drawbacks of the settling basin is that during cleaning, when accumulated solids are removed, many of the solids are resuspended in the water flow. This is exacerbated if the solids are subject to rotting/fermentation and bacterial growth. Abalone effluent solids were therefore measured for nitrogen composition, and for fixed and volatile solids. Volatile solids and nitrogen would contribute to the bacterial load and rotting potential of accumulated solids.
Effluent solids were found to have 70 ppm total ammonia nitrogen and non detectable nitrite and nitrate nitrogen, while Kjeldahl nitrogen (including nitrogen bound as protein) was found to be 2.9% of dry matter solids. Fixed solids, or ash, were found to be 38.4% of dry matter solids, with volatile solids making up the remaining 61.6% of dry matter solids.
It was originally hypothesized that because abalone effluent solids are principally derived from digested kelp forage, and because kelp forage is high in ash, that the high proportion of ash would reduce the putrescence of abalone solids. However with more than half of the settleable solids found to be volatile, and with available nitrogen of nearly 3% of dry matter, there is potential for abalone solids to rot in a settling pond and contribute to the resuspension of previously removed effluent solids.
Because abalone effluent solids settle slowly, are small, and are composed of a significant proportion of volatile dry matter, it is concluded that a settling pond would be an impractical method for abalone effluent solids removal. Large required footprint, low overall effectiveness, and likelihood of issues with resuspension of solids are the principal reasons to recommend against use of such a unit.
Alternative solids removal devices, such as media filters or drum filters, would likely be more effective and use less square footage; however tend to be much more expensive.
The tangible benefits of this study are the determination of effluent solids criteria. Kelp is used as forage for California abalone producers, and therefore the findings of this study are likely to be immediately transferable to producers wishing to reclaim effluent water for secondary usage. Design of a reclamation unit is subject to available resources; primarily available space and cost, and weighed against the performance of the unit. Settling ponds are attractive units due to their low cost of installation and operation if they are expected to be effective. However, this study indicates that the space requirement and probable ineffectiveness of a settling pond would make a more expensive filtration unit a better choice for abalone effluent reclamation.
For our facility, moving ahead with a reclamation and integration strategy will be able to plan accordingly in terms of solids removal from primary effluent. The benefits of such a strategy would be an increase in production without an increase in overall seawater pumping.
The findings of this study have not been directly adopted by other California abalone producers. It is recommended that other producers wishing to reclaim abalone effluent or address waste management use an intensive filtration unit rather than a sedimentation basin.
California abalone producers were highly supportive of this type of study, and of the need for collaborative research that promotes sustainability of the industry. It is generally accepted that integrated production and advances in husbandry techniques are beneficial to all California abalone producers. Many other producers are planning or actively pursuing diversification plans, which may benefit from reclamation or refined waste management strategies.
Additionally there was a very positive reaction from California aquaculturists in general about the Western SARE program and the availability of funds to complete these types of studies.
Study findings are the very surface of development of a complete waste management strategy. A continuation of this study should address aspects of water chemistry particular to the effluent stream that influence the feasibility of reuse, for example the alkalinity of effluent waters and their evaluation for re-use in shellfish culture, or the nutrient or micronutrient resources available for algal culture. Further, studies into effluent reclamation and reuse should be expanded to include other aquaculture species, or integration with non-aquaculture production, for example the use of freshwater effluent to support vegetable production.
The findings of this study were presented at the Aquaculture America conference in Las Vegas, NV, February 13-16 2005. (Available with the written report.)