water quality parameters were measured again in all 15 tanks 26 h, 44 h and 50 h after the sludge was removed from the ‘sludge-remove’ treatment. Sludge was not removed from any of the tanks from 0 h until the end of the experiment (i.e. 50 h).
At each sample time water quality variables were measured at the inflow of a single tank, since they all had a common water source, and at the outflow of each tank. TAN concentrations were determined using the phenolhypochlorite method of Solorzano (1969). Samples were kept in the dark once reagents were added and absorbencies were measured photometrically using a
spectrophotometer (Biochrom Novaspec II; Cambridge). Absorbencies were converted to TAN concentrations using a calibration equation (n = 30, R2 = 0.95) developed with stock solutions of ammonium-chloride. To correct for small differences in flow rate between tanks, net TAN production for each tank at each sample time was expressed as weight per second according to the equation:
Ap = F (Ao – Ai) (4.1) where, Ap is net TAN production (µg s-1), F is flow-rate (L s-1), Ao is outflow TAN concentration (µg L-1), and Ai is inflow TAN concentration (µ g L-1).
Percentage FAN was calculated using the recorded pH and temperature values (Bower and Bidwell, 1978). FAN concentration was calculated by multiplying TAN concentration by percentage FAN.
The pH was measured using a pH meter (YSI Inc. Model # 60/10 FT; Yellow Springs, Ohio).
Temperature was measured using an electronic thermometer (YSI Inc. Model # 55D; Yellow Springs, Ohio).
To verify that abalone biomass and individual abalone weight were equal in the ‘sludge-remove’
and ‘sludge-remain’ treatments the total weight of abalone in each basket was measured and summed to obtain the total weight per tank following the final water quality sample. Total weight
of abalone per tank averaged (± standard deviation, S.D.) 53.4 ± 1.1 kg in the ‘sludge-remove’
and ‘sludge-remain’ treatments and was not significantly different (Student’s t-test: P = 0.39).
This is equivalent to approximately 9.1 kg of soft abalone tissue per tank on a dry weight basis (Britz & Hecht, 1997; Sales & Britz, 2000). The mean individual weight of abalone in each tank was estimated from a sample of 30 randomly selected abalone per basket. Individual live weights averaged 18.4 ± 0.4 g and were also not significantly different (Student’s t-test, P = 0.75).
The cumulative weight of feed, placed into each tank from the start of the trial up until each water quality sample, was recorded for the ‘sludge-remove’ and ‘sludge-remain’ treatments. These data were compared using multi-factorial analysis of variance (ANOVA) at P < 0.05. The 44 h and 50 h water quality samples fell between feeding times and so cumulative weight of feed fed did not change from 44 to 50 h. Feed was not placed into the ‘no abalone or sludge’ treatment.
Flow rate into tanks of the ‘sludge-remain’ and ‘sludge-remove’ treatments was recorded every 4 days at mid-tide during the sludge accumulation period, and in all tanks whenever water quality samples were taken. Although the flow of seawater into the tanks varied with tidal-height it was similar between the ‘sludge-remove’ and ‘sludge-remain’ treatments during the sludge
accumulation period (ANOVA, P = 0.96) and amongst all treatments when water quality samples were taken (ANOVA, P = 0.73), with an overall mean of 0.37 ± 0.04 L s-1. Flow rate into the treatments with abalone in the tanks was also calculated taking abalone biomass into account and remained similar between these treatments with a combined mean of 25.7 ± 3.0 L kg abalone-1 h-1 (ANOVA, P = 0.49).
The dry weight of sludge in tanks of the ‘sludge-remove’ and ‘sludge-remain’ treatments at 0 h was estimated by collecting and drying the sludge siphoned from four tanks of the ‘sludge- remove’ treatment. The sludge was siphoned with seawater from these tanks into separate tanks and the volume recorded. The resulting sludge solution was stirred into a homogenous suspension and a 0.300 L sample was collected from each tank. The solids were filtered from these samples onto Whatman® 185 mm qualitative circles (Cat. No. 1001 185, porosity 11 µm) that had
previously been dried at 60 ºC until a constant weight (0.1 mg) was attained. The Whatman® circles with sludge were dried again and re-weighed, and the dry weight of sludge taken from each replicate of the ‘sludge-remove’ treatment was calculated using the equation:
S = (WF – WI) VT / VS (4.2) where, S is dry weight (g) of sludge, WF is final dry weight (g) of the Whatman circle, WI is initial dry weight (g) of the Whatman circle, VT is volume (l) of the sludge solution and VS is volume (l) of the sample.
At the end of the trial, samples of the sludge was siphoned from each of the five ‘sludge-remain’
tanks and collected using a 10 µm sieve. The solids were centrifuged from these samples at 2000 rpm and nitrogen content was determined using the Dumas combustion method (Williams, 1984) and a LECO FP2000 Nitrogen Analyzer (St. Joseph, Michigan).
Net TAN production, percent FAN, FAN concentration and temperature data for the ‘sludge- remove’ and ‘sludge-remain’ treatments were compared using multi-factorial ANOVA and Fisher’s LSD test (P < 0.05), over the experimental period, to test the null hypotheses that (1) production of total ammonia, (2) percentage of total ammonia in the free ammonia form and (3) concentration of free ammonia were similar in H. midae farm tanks from which sludge was removed to those from which it was not. Assumptions of normality and homogeneity of variance were checked using the Shapiro-Wilk W test and Levene’s test.