Yoram Avnimelech

Carbon/nitrogen ratio as a control element in aquaculture systems

Abstract Controlling the inorganic nitrogen by manipulating the carbon/nitrogen ratios is a potential control method for aquaculture systems. This approach seems to be a practical and inexpensive means of reducing the accumulation of inorganic nitrogen in the pond. Nitrogen control is induced by feeding bacteria with carbohydrates, and through the subsequent uptake of nitrogen from the water, by the synthesis of microbial proteins. The relationship among the addition of carbohydrates, the reduction of ammonium and the production of microbial proteins depends on the microbial conversion coefficient, the C/N ratio in the microbial biomass, and the carbon contents of the added material. The addition of carbonaceous substrate was found to reduce inorganic nitrogen in shrimp experimental tanks and in tilapia commercial-scale ponds. It was found in tilapia ponds that the produced microbial proteins are taken up by the fish. Thus, part of the feed protein is replaced and feeding costs are reduced. The addition of carbohydrates, or the equivalent reduction of proteins in the feed, can be quantitatively calculated and optimised, as shown here. Approximate parameters were used in this work. Additional research in this field should be directed at gathering the precise data needed for the exact planning of feed composition.

Shrimp and fish pond soils: processes and management

Abstract The pond bottom soil and the accumulated sediments are integral parts of ponds. Concentrations of nutrients, organic matter and microorganism density in the pond bottom are several orders of magnitude greater than in the water. The accumulation of organic sediments may limit pond intensification. The intensive organic matter degradation at the pond bottom and high sediment oxygen demand exceeds the oxygen renewal rate. This leads to the development of anoxic conditions in the sediments and at the sediment–water interface. A series of anaerobic processes, affected by the redox potential of the system, are taking place. A large number of potentially toxic materials are generated. Among those are organic acids, reduced organic sulfur compounds, reduced manganese and sulfides. Shrimp, as animals that normally live on or near the bottom, are exposed to conditions on the pond bottom. Exposure to toxic materials endanger the well being of the cultured shrimp. Reduced feeding, slower growth, mortality and possibly higher sensitivity to disease are reported. The rational use of aerators to minimize the area of sludge accumulation, construction of ponds to trap sludge, stirring sediments, chemical poising of the redox system and environmentally accepted treatment and reuse of drained sediments are means to control the conditions at the pond bottom.

Water content, organic carbon and dry bulk density in flooded sediments

Abstract Several basic properties of pond bottom soil are shown to be related, a relation that enable to evaluate pond bottom soil characteristics through the determination of one parameter (e.g. soil moisture). In addition, these relationships give some insight into the properties of flooded sediments. Unlike terrestrial soils, made of gas, liquid and solid phases, flooded sediments are made practically of only two phases, liquid and solid. Since all voids are filled with water, it is possible to evaluate soil porosity and bulk density, directly from the moisture content of the soil, a property easily determined. The correlation between bulk density and organic matter was tested in six different systems (n=868), including rivers and fish pond sediments in Israel, fish pond sediments in Alabama, USA, and Abbassa, Egypt, lake sediments in New Zealand, alpine lake sediments in Colorado, USA, and sea floor sediments from the Northwest African continental slope. Sediment bulk density was inversely related to the organic carbon concentration. The regression for all the data points was: Bulk density ( g/cm 3 )=1.776−0.363 Log e OC (R 2 =0.70) where OC is the organic carbon concentration (mg/g). The relationship between sediment moisture, bulk density and organic carbon described here can be used as a simple means to estimate the organic content and bulk density of flooded mineral soils by a simple determination of a sediment water content.

Total nitrogen analysis in water, soil and plant material with persulphate oxidation

Abstract A method for the determination of total (Kjeldahl) nitrogen in soil and water samples is presented. The sample is digested with a potassium persulphate solution in an autoclave. Subsequently, nitrates and nitrites are reduced to ammonium with Devarda alloy and ammonium determined colorimetrically. The method was found to be accurate, reproducible and efficient.

Studies on slow release fertilizers: I. Effects of temperature, soil moisture, and water vapor pressure.

Using first-order kinetics we describe the release of nutrients by coated slow-release fertilizer (SRF). Plotting the logarithm of the concentration of intact fertilizer in the soil [log(Q0 - Qt), where Qt, is the amount applied and Qt, is the amount released at time t] versus the time yielded a straight line, as predicted. The rate constants of SRF release at different temperatures are linearly related to the water vapor pressure: K = A · Pw + B where Pw is the vapor pressure and A and B are constants. We conclude that the rate-determining step in the nutrient release out of SRF is the migration of vapor from the soil into the fertilizer granule. The very mild effect of soil moisture on the rate of nutrient release is consistent with the proposed mechanism. We discuss the possibility of predicting nutrient release from SRF using the derived relationships.

Stability Indexes For Municipal Solid Waste Compost

Compost was prepared from the organic fraction of the municipal solid waste stream, using a windrow process. Stability was defined using rather simple tests. The pH was acidic at the beginning and stabilized near a neutral range. Electrical conductivity (EC) dropped and stabilized near a value of approximately 50 percent of the initial reading. The drop in EC was not due to leaching but to decomposition of organic acids. Another sensitive index was NH4 concentration. It decreased sharply from initial levels of a few hundred ppm to a stable value of about 50 ppm. The stability point as determined by simple chemical analyses of the aqueous extract of the compost correlated well with the decrease of the organic matter decomposition rate.

Biodegradation kinetics of hydrocarbons in soil during land treatment of oily sludge

This study focuses on the processes influencing hydrocarbon residue persistence in soil, following land treatment of refinery oily sludge. Treating sludge applied to soil resulted in 70% to 90% degradation of total petroleum hydrocarbon (TPH) during 2 months, regardless of their initial concentrations (9 to 60 g/kg soil). Kinetic analyses performed on TPH degradation, in laboratory and field systems, revealed a degradation pattern characterized by two consecutive first-order kinetics reactions in all experimental settings. The first stage lasted about 3 weeks and was characterized by a temperature dependent rate constant of 0.047 day−1 at 24°C. That value was comparable to the rate constant obtained when combining the individual rate constants of the saturated, aromatic, asphaltene and polar fractions. The subsequent slower stage rate constant was 0.012 day−1, insensitive to temperature and to hydrocarbon composition. The transition between the two stages (about 21 days) was independent of the experimental temperature and the biodegradation extent during the first stage. It was concluded that the extent of residual accumulation in the soil was determined by the biodegradation efficiency during the first three weeks of treatment when biological processes dominated. During the following period, abiotic processes leading to reduced bioavailability of the TPH were limiting the degradation rate. Practically, as the first few weeks of treatment determine its efficiency, efforts to enhance the biological activity should be directed to that period.

The effect of local anaerobic conditions on growth retardation in aquaculture systems

Abstract The existence of anaerobic zones in a densely stocked fish pond with circulated water was determined by measuring the rate of nitrate reduction following addition of NO3− to the pond. Rate of denitrification during the first 3 months after stocking was very low — 0 to 0.05 mg NO3-N/l·h−1. Later, denitrification rates rose to an average value of 0.2 mg NO3-N/l·h−1, even though the pond was aerated continually and the water as a whole was aerated. Denitrification occurs due to development of anaerobic zones somewhere in the pond. Fish growth was good during the first 3 months and was retarded when anaerobic zones had developed. Possibilities of using nitrate additions to ponds, both to indicate local reducing conditions and to control and prevent such conditions, are discussed.

The effect of drying on the colloidal properties and stability of humic compounds

It is indicated here that organic macro molecules in the soil are aggregated due to bonds, presumably hydrogen bonds, formed in the presence of water. When the soil is dried, this structure is broken and the stability of the organic matter decreases. This process is not an instantaneous process and apparently some relaxation period is needed for a complete dispersion of the organic matrix. An appreciable fraction of the organic molecules is hindered, even from contact with the solution in the aggregated structure, as indicated by the lower acidity in the wet soil. This steric hinderence seems to be one of the important factors contributing to the stability of humic compounds.

Rates of organic carbon and nitrogen degradation in intensive fish ponds

Abstract Water quality in intensive aquaculture systems is to a large extent controlled by the microbial biodegradation of organic residues. The ability to simulate processes in such systems depends on the availability of data on the rates of these processes. The first order kinetic rate constants for both organic carbon and organic nitrogen degradation in fish ponds were evaluated. The rate constants were computed by either relating the measured rate of disappearance of the substrate concentration or by the comparison of the measured with a theoretically derived evolution pattern of organic carbon or nitrogen concentrations in model ponds. The organic carbon degradation rate constant was found to be 0.15/day. The organic nitrogen degradation rate constant was expected, theoretically to be about 0.06/day and was found experimentally to be in the range of 0.05–0.09/day. These values seem to be reasonable approximations for constantly aerated-mixed ponds, at a temperature range of 20–30 °C. Organic matter previously metabolized in anoxic sites at the pond (e.g. resuspended sediments), has a degradation rate of ca. 0.6/day, and can thus consume large amounts of oxygen in the water body.