Effluent characteristics of dairy shed oxidation ponds and their potential impacts on rivers

Transcription

1 New Zealand Journal of Marine and Freshwater Research, 1989, Vol. 23: / $2.50/0 Crown copyright Effluent characteristics of dairy shed oxidation ponds and their potential impacts on rivers CHRISTOPHER W. HICKEY JOHN M. QUINN ROBERT J. DAVIES-COLLEY Water Quality Centre Division of Water Sciences Department of Scientific and Industrial Research P.O. Box , Hamilton, New Zealand Abstract The effluent characteristics of 11 dairy shed oxidation ponds designed to national specifications were examined. Measurements covering a wide range of parameters were made monthly over at least 1 year in ponds from two regions (Manawatu and Southland) and covering two types of farms: town milk supply (non-seasonal) and daily factory (seasonal). There was considerable variation in effluent composition within ponds with time and between different ponds. Biochemical oxygen demand (BOD) concentrations (overall median 98 g nr 3 ; 3-fold range of individual pond medians) showed less variation between ponds than suspended solids (SS) concentrations (median 198 g nr 3 ; 9-fold range). Available nutrient levels were very high (e.g., an overall median of 12.2gm 3 for dissolved reactive phosphorus and 75.0 gm 3 for ammonium (NH 4 -N)). The ammonia represents a 4-fold higher level of potential oxygen demand than the measured BOD. Faecal coliforms (median (100 ml) 1 ) showed large variability both within and between ponds, with higher levels in town milk supply ponds. Differences in pond effluent characteristics could not be attributed to influent loading as apercentage of design or to the significant temperature difference between regions. Received 7 November 1988; accepted 9 June 1989 Maintenance of receiving waterconcentrations below existing criteria for 95% of the time would require > fold dilution for faecal coliforms (bathing criterion), > 67-fold dilution for coliforms (post treatment drinking criterion), and > 2700-fold dilution to prevent nuisance levels of algal proliferations below discharges. Most uses are accommodated provided dilution exceeds 250-fold requiring a minimum stream flow of m 3 s^for a220-cow herd. Keywords effluent; wastewater; oxidation ponds; lagoons; dairy sheds; water quality; rivers INTRODUCTION The dairy industry is a major producer of agricultural wastes in New Zealand. About half of the more than dairy sheds with water rights discharge effluenttorivers(hickey&rutherfordl986).dakers & Painter (1983) reported that 14.5%, or over 2000 farms, used lagoon treatment systems in However promotion of oxidation pond treatment systems in recent years has increased their number substantially. For example, the effluents from c. 50% of the 3500 dairy sheds in the Waikato Catchment Board area are now treated by ponds (pers. comm. G. Woodward). Dairy shed pond effluents represent a potentially large impact on receiving waters, especially small streams which may receive multiple discharges. In particular the organic matter, nutrients, and suspended solids, acting either individually or in combination, discharged into rivers cause concern. Organic loading may result in increased water and sediment oxygen demand resulting in river deoxygenation (Hickey 1988). Nutrient additions may promote nuisance growths of algae and macrophytes, often resulting in depletion of nighttime dissolved oxygen (DO) levels and ph excursions (Rutherford et al. 1987). Nitrification of ammonia may also decreaseriver DO (Cooper 1984). Ammonia may also reach concentrations which are toxic to fish

2 570 New Zealand Journal of Marine and Freshwater Research, 1989, Vol. 23 and invertebrates present in receiving waters. Suspended solids may significantly reduce water clarity, and by settling may result in smothering of benthos. Water colour may be altered by dissolved organic material as well as suspended solids present in the effluent. Currently there is a lack of information on both the effects of individual dairy shed pond effluents on receiving waters, and their general effluent characteristics upon which confident predictions of their likely environmental impact can be made. A programme of research on the ecology of New Zealand rivers at the Water Quality Centre, has as an objective appraising the nature and extent of ecological effects of wastewater discharge. Thus, it is pertinent to survey the effluent characteristics of ubiquitous point-source discharges such as dairy shedanddomestic sewage oxidation ponds. Domestic sewagepondeffluentcharacteristics will be discussed in a subsequent publication (Hickey et al. 1989). Information on dairy shed oxidation ponds is presently held only in the data files and internal reports of water boards and a conference proceedings (McFarlane 1983). The present paper presents the first national perspective on dairy shed oxidation pond characteristics based on summary of large data sets collected by two catchment boards covering much of the range of New Zealand geographical conditions. The objectives of this study were: 1. To document the effluent characteristics (mean, standard deviation, median, range): nutrients, biochemical oxygen demand (BOD), suspended solids (SS), and coliforms in a range of dairy shed oxidation ponds. 2. To determine whether seasonal or geographical differences, or influent loading affect effluent character. 3. To investigate relationships between constituents of pond effluents. 4. To assess the likely effects of dairy shed oxidation pond effluents on rivers in terms of available criteria for water uses. METHODS Study ponds Data were supplied by the Manawatu and Southland Catchment Boards on a total of 11 dairy shed oxidation ponds. The selection of each pond was conditional upon having been designed to national specifications (Ministryof Agriculture andfisheries (MAF) 1983). The design guidelines suggestarange of loading rates to the anaerobic pond based on the number of animals and the geographical area. Loadings in the Southland (46 S) and Manawatu (41 S) regions are 20 and 24 g BOD 5 nr 3 day- 1, respectively. Aerobic pond loading is equivalent in all areas at 8.4 g BOD 3 m~ 2 day- 1 or 3.25 m 2 cow 1. The anaerobic pond loading rates were calculated from the measured pond volumes (all Manawatu and one Southland pond (S2)) or as built volumes (Southland ponds S1, S3-S5), and the cow numbers, assuming 90 g BOD 5 cow - 1 day" 1 (MAF 1983). The aerobic pond loading rates were calculated from the measured pond surface areas and assuming that 70% of the BOD 5 loading to the anaerobic pond was removed (MAF 1983; Dakers et al. 1984). The pond hydraulic residence times were calculated by dividing the pond volumes, derived from measured pond dimensions (Manawatu data) or as built dimensions (Southland) by the flow, calculated from the cow numbers assuming an average daily flow for modern dairy sheds of m 3 cow - 1 day 1 (Drysdale & Painter 1983; Warburton 1983; Vanderholm 1984). All of the ponds were in wind-exposed positions and appeared to be well maintained. Only pond S2 had developed a thick crust of dried material on the anaerobic pond. Sampling Of the six Manawatu ponds, three were for nonseasonal town supply dairy farms and the others supplied processing factories (seasonal). The pond effluents were sampled monthly between 0900 and 1000 h from August 1985 to July Temperature, conductivity, ph, DO, BOD 5, SS, total phosphorus (TP), dissolved reactive phosphorus (DRP), ammonia, nitrate, coliforms, and faecal coliforms were analysed. Five Southland ponds, only two of which were for town supply farms, were sampled from September 1976 to January 1986, with the majority of samples taken during monthly surveys in The variables analysed were: temperature,conductivity,ph,bod, SS, and ammonia. All samples were taken either from the discharge pipe or from immediately adjacent to the discharge structure from a depth of 0.2 m, using 2-litre plastic bottles. Bacteriological samples from Manawatu were taken in pre-sterilised glass bottles and chilled during transport (all other samples were notchilled). The maximum time delay before starting analyses was 4 h.

3 Hickey et al. Dairy shed oxidation ponds 571 Analyses and measurements Field measurements of DO and temperature (YSI, Model 54) were made for the Manawatu samples. Only temperature (mercury in glass thermometer) measurements were made in the field on Southland ponds. Samplesfordissolvednutrientswerepre-filtered through 0.45 (im cellulose acetate membrane filters (Sartorius). All chemical analyses were by APHA methods (APHA 1981, for Manawatu; APHA 1985, for Southland), with the exception of Southland NH 4 -N analyses which were by ion-selective electrode at thecatchmentboardlaboratories. DO measurements for the 5-day BOD test were made using an aircalibrated DO meter. Nitrification was not inhibited in these BOD tests. Consequently, some proportion of the 5-day BOD was probably nitrogenous. Coliform numbers were determined using the MPN method. Total presumptive coliforms used incubation in minerals modified glutamate media (35 ± 0.5 C for 48 ± 2 h), with confirmation of positive tests by further incubation in lauryl sulphate broth ( C for h). Faecal coliforms were determined by inoculation from the positive presumptive tests to lauryl sulphate broth (44.5 ± (WC for 24 ± 2 h). These methods are in accordance with the New Zealand Microbiological Society (1976) except that a 35 C incubation temperature was used rather than the 37 C specified. RESULTS Pond loadings The 11 ponds whose effluents were studied were each designed to MAF specifications (MAF 1983) for two-stage (anaerobic followed by aerobic) effluent treatment. Pond data and loading calculations are shown in Table 1. Influent loadings (based on cow numbers) were appreciably exceeded (by >20%)inonlyoneoftheponds(S2). In the remainder the loading rates were generally less than recommended (55-120% foranaerobicponds and % for aerobic ponds). Thus most of the ponds were correctly loaded, or slightly underloaded, relative to the design criteria. Calculation of the average retention time gave values ranging from 56 to 209 days (Table 1). The median retention time was 115 days. Variability of effluent characteristics Figure 1 shows the distributions of selected key variables for all data taken together as cumulative frequency curves. Statistics calculated from these distributions are summarised in Table 2. Only temperature and conductivity had normal distributions. Although log transformation of the other Table 1 Dairy shed oxidation pond physical characteristics. Type: T, town supply; S, seasonal supply; Pond: M, Manawatu; S, Southland; HRT, hydraulic retention time; ND, no data. See text for method of calculation. Pond Ml M2 M3 M4 M5 M6 SI S2 S3 S4 S5 Type T S S T S T T S S S T BOD, loadings %ofmaf(1983) a Pond influent flows'" (m'clay- 1 ) Total pond surface area (m 2 ) Storm-water surface area HRT (m 2 ) (days) d ND ND ND ND ND Time since built c (years) " Based on cow numbers and pond dimensions; b Assuming 0.08 m 3 cow" 1 discharged to ponds (Drysdale & Painter 1983; Warburton 1983; Vanderholm 1984) c Time to middle of sampling period (1983 for Southland ponds when most data collected) d Southland ponds were corrected for sludge accumulation in the first pond by assuming 10% depth reduction per year as measured by Warburton (1983)

4 572 New Zealand Journal of Marine and Freshwater Research, 1989, Vol. 23 i_ 3 -» (0 ' ' 2 Q) > z ^ c u o ^ ^ < p es«8^100 r 2 r S 10 (0 V o o = E / Cumulative non-exceedance (%) Fig. 1 Cumulative frequency distribution curves for effluent characteristics from dairy shed oxidation ponds. (Note: A-D normal ordinate; E-H, log ordinate.)

5 Hickey et al. Dairy shed oxidation ponds 573 B s * 4 H S i > ^300 " 5 E 1» 200 5^100 m m " ct800 i ~ 'ra if 300 E K (A 106 ItI It ~ M1 M2 M3 M4 M5 M6 S1 S2 S3 S4 S5 M1 M2 M3 M4 M5 M6 Site code Site code Fig. 2 Comparison of effluent characteristics for Mimawatu (M) and Southland (S) dairy shed oxidation ponds. Boldface indicates a town supply (non-seasonal) pond. Box indicates interquartile range, the bar the median value and "whiskers" 1.5 X interquartile range; with outliers (O > 1.5 X interquartile range) and extreme outliers (* > 3.0 X interquartile range). Shaded portions indicate approximated 95% confidence intervals (1.58 X interquartile range/vn) for the comparison of medians (from Velleman & Hoaglin 1981). variables produces fairly symmetric distributions (Fig. 1), only in one case (for suspended solids) did the distribution fit a log-normal function based on the skewness and Shipiro-Wilk tests (P<0.1). Other variables (e.g., faecal conforms) were negatively skewed when log-transformed. Variations both within individual ponds with time, and between the ponds, contributed to the total variability of individual constituents in the combined data set (Table 2, Fig. 2). Variability (5 percentile- 95 percentile range) was < 10-fold for conductivity (8-fold), ph, BOD (8-fold), TP (4-fold), and DRP (4-fold). Greater data ranges, but < 100-fold, were observed for DO (10-fold; this variability cannot be attributed to the effect of sampling time, since all ponds were sampled largely between 0900 and 1000 h); SS (16-fold) and NH 4 -N (27-fold). Variability of > 100-fold occurred for N0 3 -N (1270- fold), coliforms (664-fold), and faecal coliforms (2700-fold).

6 574 New Zealand Journal of Marine and Freshwater Research, 1989, Vol. 23 Table 2 Effluent characteristics for the combined data of 11 dairy shed oxidation ponds. S, Southland; M, Manawatu; Cond., conductivity; DO, dissolved oxygen; BOD, biochemical oxygen demand; SS, suspended solids; TP, total phosphorus; DRP, dissolved reactive phosphorus; NH 4 -N, ammonium nitrogen; NO 3 -N, nitrate nitrogen; Coli, total coliforms; Fcoli, faecal coliforms. Variable Geometric No. of (units) mean Median 5 percentile 95 percentile samples Region Temp. ( C) Cond. (ms nr 1 ) ph DO (g nr 3 ) BOD (g nr 3 ) SS(gnr 3 ) TP(gm- 3 ) DRP(gm" 3 ) NH 4 -N (g m' 3 ) NO,-N (g nr 3 ) Coh (100 ml)" 1 Fcoli (100 ml)" t 157t * 25.7* t Normal distribution based on skewness and Shipiro-Wilk tests (P < 0.1) % Log-normal distribution S,M S,M S,M M S,M S,M M M S,M M M M Table 3 Summary of effluent characteristics for individual dairy shed oxidation ponds. (See Table 2 for abbreviations.) Variable (units) Temp. ( C) Cond. (ms nr 1 ) ph DO(gnr 3 ) BOD (g nr 3 ) SS(gnr 3 ) TP(gm- 3 ) RDP(gm" 3 ) NH 4 -N(gnr 3 ) NO.-N (g m' 3 ) Colii(100 ml)" 1 Fcoli (100 ml)" Geometric mean* (10/1 l) d (10/11) (4/11) (4/6) (7/11) (7/11) (5/6) (4/6) (3/11) (2/6) (4/6) (4/6) % CV b (maximum) (19.8) (10.3) (4.6) (77.4) (21-8) (18.0) (12.1) (15.4) (41.9) (330) (25.3) (24.4) Sample no. required to estimate 0 mean within: 10% a Average (geometric mean) of individual pond geometric means b Average (geometric mean) of individual pond CV (standard deviation X 100/mean) values c Calculated from Elliott (1977:129) d Normal or log-normal distributions/number of ponds (i.e., fraction of ponds with normal or lognormal distributions) 20% Some characteristics (e.g., SS) varied widely between individual ponds (Fig. 2). This may be the main reason for the non-normality of the combined data sets. Wide variations in some characteristics were attributable to both regional and individual pond differences. Logarithmic transformation gave near-normal distributions for a greater number of individual pond variables (Table 3) than for pooled data (Table 4). Because some distributions were not normal or log-normal, comparisons are based mainly on median values, although geometric mean values are given in tables since parametric estimators are more useful for calculation of monitoring requirements. The variability of determinands within individual ponds is shown in Fig. 2 and summarised in Table 3. Temperature, ph, and DRP each showed relatively similar median values in all the ponds (< 2-fold variation, Fig. 2). Medians of conductivity, DO, BOD, and TP ranged 3-fold. Greater ranges in the

7 Hickey et al. Dairy shed oxidation ponds 575 medians were observed for SS (9-fold), NH 4 -N (8-fold), NO3-N (217-fold), coliforms (14-fold), and faecal coliforms (26-fold). For design of monitoring programmes an indication of within-pond variability (with time) is useful. A broad impression of this within-pond variability is obtained from Fig. 2 and Table 3 presents the averages of the coefficients of variation (C V) values for the individual ponds. The coeff icients of variation for most of the parameters was between 10 and 15%, with DO greater at 50%. The number of samples required to obtain an estimate of the mean with a precision of 10% was generally about 10. Geographic variability Variations on a regional basis contributed to the variability of individual components for the combined data set (Table 2). Data separated by region showed significant differences (P < 0.05) between many of the variables as shown in Table 4. The median loading to anaerobic and aerobic ponds in Southland was higher, though not significantly (P > 0.05), than to those in Manawatu. The retention time was not significantly different between regions (median=115 days), however, there was a greater range of values in Southland. The median temperature of the Southland ponds was 5.2 C lower than that of the Manawatu ponds (P < 0.05). Significantly lower values for Southland ponds were also observed for conductivity (53% of Manawatu values, P < 0.05), suspended solids (57%, P < 0.05), and ammonia (14%,P< 0.05). There were no significantdifferences between BOD concentrations (P > 0.05; Fig. 2, Table 4). These results indicate that Southland ponds are producing lower SS concentrations, an observation which cannot be attributed to lower pond influent loadings than Manawatu ponds (see Table 1), but may be associated with differences in temperature, climatic, or operational variables. The type of pond, whether town supply, and hence operational throughout the year (boldface site codes on Fig. 2), or seasonal factory supply, may influence the pond effluent characteristics. In the Manawatu the three town supply ponds had the lower median DO and nitrate concentrations and the higher median coliform and faecal coliform levels than the factory supply ponds. However, these differences may be related to the relatively low age of these ponds rather than differences between seasonal and town supply operations. Conductivity, S S, and TP were also higher in two of the three town supply ponds than in factory supply ponds (Fig. 2). Seasonal variability Seasonably of pond effluent characteristics was examined by separation of the regional pond data into "summer" (December-February inclusive) and "winter" (June-August inclusive) periods. The median winter temperature in Manawatu was 12 C increasing to 22 C in summer, with a Southland winter median of 6 C increasing to 17 C in summer (Table 5). These seasonal and regional temperature differences were not generally reflected in pond nutrientconcentrationchanges.notably.bod values were similar between seasons and between regions. Within a region only SS concentration changed significantly with season (P < 0.05; non-parametric test for comparison of medians, see Velleman & Hoaglin 1981), with lower values in winter in Southland (Table 5); however, Manawatu SS concentration also decreased in winter. Between regions, conductivity values were significantly lower in Southland for both seasons, whereas NH 4 -N values were also lower, but only in summer. SS values were lower (though not significantly, P > 0.05) in Southland than in Manawatu for both seasons. Notably, conductivity, BOD, and SS did not show significant seasonal changes related to either Table 4 Comparison of regional effluentcharacteristics for combined data from Manawatu and Southland ponds. (See Fig. 2 for abbreviations and details of 95% CI calculation; NS, not significant.) Variable Load anaerobic (% of design) Load aerobic (% of design) Retention time (days) Temp. ( C) Cond. (ms nr 1 ) BOD (g nr 3 ) SS(gm- 3 ) NH 4 -N (g nr 3) t From Velleman & Hoaglin (1981) Manawatu median (95% C.I.) (53-101) (52-92) (92-143) ( ) ( ) (92-96) ( ) ( ) Southland median (95% C.I.) (54-128) (28-142) (51-179) ( ) ( ) (79-81) ( ) ( ) Significancet NS NS NS P < 0.05 P < 0.05 NS P < 0.05 P<0.05

8 576 New Zealand Journal of Marine and Freshwater Research, 1989, Vol to 3 oa. o a 15 o B E D) Q m so y=0.15x-9.3 r 2 = Conductivity (ms rrr 1 ) \ y=0.13x+58.0 r 2 = Suspended solids (g nv 3 ) Fig. 3 Correlations between effluent characteristics for median valuesfromdairy shed oxidation ponds: A, TP and conductivity (Manawatu ponds only); B, BOD and SS (all ponds). town supply or factory pond types. This suggests that, regardless of whether seasonal variability in influent load occurs or not, effluent characteristics for an individual pond are not related to seasonal or influent load reductions. Relationships between effluent characteristics Correlations (Spearman Rank) between variables were investigated for median values from all ponds, individual data from all ponds and individual values for ponds grouped by type. Correlation matrices for median values and individual data are shown in the Appendix. Correlations between temperature, BOD, and SS were investigated for all data, for individual ponds, and for ponds grouped by type. Temperature showed no significant relationship with any of the variables (JP > 0.05). This suggests, somewhat surprisingly, that seasonal or geographically related temperature differences are not primary driving forces affecting pond performance, since loading to the aerobic pond was comparable in the two regions (Table 3). It also suggests that covarying independent factors, notably light (irradiance), do not have major influences on pond effluent characteristics. Correlations between % of pond influent design loading (to anaerobic and aerobic ponds), retention time, and median values for individual pond conductivity, BOD, SS, coliforms, and other variables were investigated. S urprisingly, neither pond loading nor retention time was significantly correlated (P > 0.05) with BOD, SS, coliforms, or any of the variables, with the exception of NH 4 -N which was significantly correlated with anaerobic load (see Appendix). Significant(P<0.05)strongrelationships Table 5 Comparison of seasonal effluent characteristics for combined pond data from Manawatu and Southland regions. Bold values indicate significant differences within regions. See Fig. 2 for details of 95% C.I. calculation, and Table 2 for abbreviations. -, no data available. Variable Summer (n = 18) Manawatu Winter (n=21) Summer (n=25) Southland ([n Winter = 12) Significance between regionsf Summer Winter Temp. ( C) 22.0 ( ) 11.5 ( ) 16.5 ( ) 6.0 ( ) P<0.05 /><0.05 Cond. (ms nr 1 ) 256 ( ) 229 ( ) 130 ( ) 116 (69-163) i><0.05 P<0.05 BOD (g m" 3 ) 98 (54-142) 91 (70-112) 87 (60-114) 77 (31-123) NS NS SS(gm" 3 ) 363 ( ) 226 (61-391) 245 ( ) 110 (68-152) NS NS NH 4 -N(gnr 3 ) 95 (79-111) 81 (39-122) 62 (48-76) 55 (31-78) P<0.05 NS Fcoli (100 ml)- 1 ( ) ( ) t From Velleman & Hoaglin (1981)

9 Hickey et al. Dairy shed oxidation ponds 577 were found between conductivity and each of BOD (r = 0.83), SS (r = 0.89), and TP (r = 0.94; see Appendix). This suggests that conductivity provides a useful indication nutrient levels (Fig. 3A). BOD was strongly related with SS (r=0.89; see Appendix, Fig. 3B) and TP (r = 0.94). Faecal coliforms were a large proportion of total coliforms (r = 0.89). Ratios of FC/TC were 0.78 (95% confidence interval ) for medians, and 0.78 ( ) for individual values. Relationships between variables were also investigated for individual data values. Significant (P < 0.01) strong relationships were found between conductivity and SS (r=0.82), and TP (r=0.90; see Appendix); TP and SS (r = 0.83); coliforms and faecal coliforms (r=0.93); and weaker relationships between conductivity and BOD (r=0.55); BOD and SS (r=0.47), and TP (r=0.53); faecal coliforms and SS (r = 0.48), and NH 4 -N (r = 0.56). DISCUSSION Ten of the 11 ponds selected for this study were loaded at, or somewhat below, the MAF specifications for two-stage dairy shed oxidation ponds (MAF 1983). On this basis some degree of uniformity of wastewater treatment might have bee n expected; however, given the presence of secondary ponds of varying sizes, retention time may be expected to relate better to effluent constituent concentrations. We were surprised, therefore, to find wide variability in effluent character between ponds and no significant relationships with pond load (which varied 4-fold and 2-fold to the anaerobic pond and aerobic pond respectively, Table 1) or retention time (which varied 4-fold). Much of the variation may be the result of the stochastic character of the major meteorological forcing variables, including light, wind, and temperature resulting in relatively unstable conditions. For example, the settling of solids on the bed is likely to be a major net removal mechanism, however, wind mixing is likely to result in frequent resuspension of material. The complex interaction anticipated between biochemical and environmental processes as regards suspended solids is also expected to contribute to effluent variability. Waste solids are mineralised by bacteria, whereas algal growth produces new solids. These are effectively antagonistic processes in terms of paniculate matter removal. Much of our difficulty in relating the variability between and within ponds to environmental factors and with loading rates probably arises from the lack of data on some likely forcing variables (notably wind and light), and the inappropriateness of a monthly sampling regime for detecting relationships between processes which operate over much shorter time scales (e.g., wind mixing and algal growth effects on SS). Clearly, more measurements covering appropriate time scales would be required to investigate the processes influencing oxidation pond effluent characteristics further. Differences in shed management practices (such as the use of different wash detergents, disinfectants, and stock drenches (some of which may be toxic to pond organisms)) may also contribute to effluent variability by influencing pond organisms. Differences in shed management practices, such as the frequency of washdown or the holding times of cows in the pond catchment areas may also influence the pond influent flows and pollutant loads for a given herd size. Thus measurement of actual influent flows and concentrations of specific constituents (e.g., BOD and SS) rather than their calculation from average data, may have yielded better relationships with effluentcharacteristics than found in this study. Nevertheless, this study did identify two ponds (M2 and M6) which had lower, and less variable, constituent concentrations (BOD, SS, and NH 4 -N) than comparably loaded ponds (Fig. 2). Both ponds also had lower conductivity than other Manawatu ponds, with one pond (M2) exhibiting significantly higher DO levels than other ponds (Fig. 2G). These differences may indicate of pond processes affecting effluent concentrations but their significance is unknown. These ponds wouldbe suitable candidates for detailed operational studies to identify reasons for their good performance in comparison with other ponds. Seasonal and regional differences Significant temperature differences between ponds suggested both seasonal and regional climatic differences (also light, wind, and rainfall) but these were not reflected in effluent character. Seasonal "summer" temperatures (December-February inclusive) were higher by about 10 C than "winter" temperatures (June-August inclusive), and there was also a 5 C regional difference between Manawatu and Southland. A 10 C temperature difference would be expected to alter biological growth and reaction rates by about a factor 2 (Gaudy & Gaudy 1981) and thus may be expected to alter pond processes. Surprisingly, no significant seasonal

10 578 New Zealand Journal of Marine and Freshwater Research, 1989, Vol. 23 response was observed for BOD, SS, or ammonia concentrations (with the exception of SS in Southland which was lower in winter, Table 5). We observed a similar lack of seasonal differences in BOD and SS in a study of domestic sewage ponds which covered a greater range of constituents (Hickey et al. 1989). This appeared to result from higher algal biomass in summer than in winter, counteracting the increased rate of mineralisation of sewage BOD and SS expected at the higher summer temperatures. The generally lower concentrations of coliforms and faecal coliforms in winter were surprising given the much lower ultra-violet light levels in winter (Nichol & Basher 1986) and contrasted with the pattern in domestic sewage pond effluents (Hickey et al. 1989). The lower winter levels in the dairy ponds were possibly associated with lower influent loadings. Ponds in Southland had significantly lower SS concentrations than those in the Manawatu (Table 4). Southland ponds also showed a lower median conductivity, only 58% on average of those in the Manawatu, implying more dilute pond waters. This is unlikely to result from lower evaporative losses nor greater volumes of rain water captured by the cowshed yards in Southland compared with the Manawatu, since rainfall is similar in the two regions on both an annual and summer basis (Finkelstein 1973; New Zealand Meteorological Service 1984). Thus the regional effects must be related to differences in management practices, and awaits further investigation. BOD concentration was not significantly different between the two regions. Notably, significant (P < 0.05) temperature differences between regions was not reflected in BOD or SS concentrations. NH 4 -N concentrations appeared to be lower in association with lower temperatures on a regional and seasonal basis. Differences between town supply and factory supply ponds were not large except for faecal coliforms, which were higher in the town supply ponds (Fig. 2L). Town supply ponds showed somewhat higher conductivities, TP, and coliform concentrations than seasonal ponds (Fig. 2); with lower DO and nitrate concentrations. The medians and geometric means obtained in these studies (summarised in Table 2) agree very closely with the average values reported by Warburton (1983) for in tensive (weekly) monitoring of a Manawatu dairy oxidation pond over 2 years. The pond studied by Warburton was highly loaded (38 g BOD nr'dajr 1 to the anaerobic pond, c.f. the conventional design value of 24 g BOD nrmay- 1 ). However, average values for BOD, SS, and NH 4 -N were within 20% of the median values reported here. Warburton measured generally higher TP and DRP and especially nitrate. The overall treatment efficiency of BOD removal was reported to be consistently high (c. 95%), with only minor seasonal fluctuations. Effluent flows Warburton (1983) provides detailed information on the effluent treatment efficiency and the water balance. Rainfall resulted in up to 2-fold increases in pond daily effluent flow rates whereas evaporative losses reduced effluent flow by 30% on average during a 40-day "dry period" (calculated from data in Warburton 1983: fig. 4). Similar losses can be calculated from December dry periods (when openwater evaporation is usually highest) using Finkelstein's (1973) open-water evaporation data and the pond inflow and surface area data in Table 1. However, evaporative losses are minimal (< 1%) in dry periods in winter (Finkelstein 1973). Overall the annualaveragerainfall (NewZealandMeteorological Service 1984) on the pond surface and storm water collection areas (Table 1) would be expected to exceed open water evaporation (Finkelstein 1973) by c. 500 mm in both Southland and Manawatu (calculated assuming that Southland pond's stormwater collection areas increase their catchments by the average area measured for the Manawatu ponds of 25% (Table 1)). This is calculated to cause a net increase in annual average effluent over influent flows (Table 1) of c. 15%. Thus, although evaporation can reduce receiving water impacts by reducing effluent flows by c. 30% in dry weather during summer (Warburton 1983), in general rainfall inputs exceed evaporative losses. For the ponds studied the median calculated pond influent flow was 17.6 m 3 day" 1 (range = m 3 day" 1, Table 1). The 15% increase in flow expected from the balance of rainfallevaporation would be expected to increase the median annual average effluent discharge to 20.2 m 3 day 1 (0.23 litres s' 1 ), whereas a 30% evaporative loss during dry periods in summer (Warburton 1983) would reduce the median discharge to 12.3 m 3 day" 1 (0.14 litres s" 1 ). Receiving water effects Oxidation pond discharges have the potential to significantly impact receiving waters by: affecting the natural biota; degrading aesthetic quality; and restricting human uses. Organic matter, ammonia

11 Hickey et al. Dairy shed oxidation ponds 579 toxicity, nutrients, and suspended solids, acting either individually or in combination, together with pathogens which may be present, can all impact receiving water quality. A summary of effluent dilutions necessary to achieve desirable receiving water concentrations with regard to different water uses for the range of variables measured is shown in Table 6. Dilutions required are calculated for both median and 95 percentile effluent concentrations, with the latter indicating a high level of environmental protection. For some variables the dilution required may vary considerably between regions (e.g., for SS (see Fig. 2E) and ammonia (see Fig. 2F)), seasons, individual ponds, and in relation to the nature of the receiving waters. Dilution required based on the 95 percentile ranged from 5-fold to over 2700-fold with the highest values being associated with the restriction of algal proliferations immediately below discharges, and with bacterial quality with respect to recreational bathing. Most other uses are accommodated providing the dilution exceeds 250-fold (i.e., flow of uncontaminated receiving water > 58 litres s" 1 for the median effluent flow of 0.23 litres s J (for a 220-cow herd and correcting for the balance of rainfall and evaporation). Discontinuity of discharges would create considerable difficulties in assessing the likely impactonreceivingwatercommunities of fluctuating constituent concentrations. Significant oxygen depletion in the receiving water is possible because of the BOD and ammonia present. Based on the results of domestic sewage oxidation pond studies, most of the BOD measured in the oxidation pond effluent would be expected to be particulate organic material (mostly algae) (King et al. 1970; Hickey & Quinn unpubl. data). The dissolved BOD as a proportion of total BOD has not been measured for New Zealand dairy shed ponds. Sewage fungus proliferation, which relates more closely to the BOD of dissolved (particularly low MW) organic material (Quinn & McFarlane 1988; in press), would generally be expected to occur only in rivers containing very high concentrations of pond effluent (e.g., BOD S > g nr 3 ), or when ponds are grossly overloaded. Table 6 Dairy pond effluent dilutions required to achieve desirable receiving water concentrations for various uses. See Table 2 for abbreviations and text for references. Variable DO BOD SS TP DRP NH.-N NH 4 -N (oxygen demand) NO,-N DIN BODandNH 4 -N (oxygen demand) Coli d Fcoli d Effluent concentration Median (g nr 3 ) C C Receiving water 95 percentue criterion (g nr 3 ) (g nr 3 ) 1.1* " (min.) 5 (max.) 10 (max.) b? (max.) 0.77 (max.) 5 (DO) (max.) (DO) ]Dilution factor required t Median >1.8 >20 >20 >1220 >97 >65 >1 >940 >83 -I >8 >35O >35 95 percentile >4.5' >50 >80 _ >1710 >248 >165 >318 >2700 >214 - >67 >2700 >270 Use biota resp. biota resp. aesthetics _ algal growths fish toxicity biota resp. algal growths algal growths biota resp. bathing drinking bathing drinking t Minimum dilutions required assuming no background "contamination" $ No criteria available;?, no known criterion appropriate for rivers 5 percentile value b Approximate level at which clarity and colour impacts on streams with relatively clear background water become "obvious" c Calculated using a factor of 4.33 X to convert NH 4 -N to oxygen demand equivalents (Wesernak & Gannon 1968) d All coliform and faecal coliform concentrations are number (100 ml)" 1

12 580 New Zealand Journal of Marine and Freshwater Research, 1989, Vol. 23 Notably, the potential oxygen demand (Table 6) of the ammonia present is 4-fold greater than that of the measured BOD. Thus for the management of river DO.bothBODandammoniamustbeconsidered (Cooper 1986). In shallow rivers the rate of DO depletion may be greatly enhanced by organisms growing attached to the bed, with associated high benthic oxygen uptake rates (Hickey 1988). When ammonia levels are high, benthic nitrification may also exert a significant oxygen demand on river DO (Cooper 1984). Management guidelines have recently been developed for river nitrogenous oxygen demand (Cooper 1986). In shallowrivers the resident nitrifying population on the river bed exerts the majority of the nitrogenous oxygen demand; planktonic nitrifiers are unimportant. Rapidremoval of NH 4 -N, with consequent DO depletion, may adversely affect stream life. Suspended solids have the potential to: degrade aesthetic quality by effects on water clarity; smother the river bed by settling; and to cause DO reductions in interstitial waters and overlying river waters by respiration and decay. Although some authors have argued that pond algae do not constitute a significant impact on DO (Gloyna & Tischler 19 80), the available evidence indicates that pond algae do not survive for any appreciable time in the receiving water, but die and degrade sometimes resulting in significant oxygen demand (King et al. 1970; Sutherland 1981; Cosser 1982). The extent of impacts on biological communities in New Zealand rivers has not been studied in detail. The effects of pond effluents on water clarity may be substantial. Based on an incremental figure of 10 g nr 3 SS representing a visibly conspicuous reduction in water clarity in small, clear streams (Davies-Colley unpubl. data), a minimum 80-fold dilution would be desirable. Considering that SS levels were 4-fold higher in some ponds (Fig. 2E), then proportionately higher levels of dilution would be required at some times. The appropriate data are not available for more detailed analysis of colour/ clarity impacts of dairy ponds. Excessive proliferations of both algae and aquatic macrophy tes can choke waterways, severely reducing their drainage capacity (e.g., Dawson & Robinson 1984) and amenity value, and causing excessive diurnal fluctuations in DO and ph (Freeman & McFarlane 1982; Quinn & Gilliland 1989). Algal slime growths were found to be the single greatest limitation to instream uses of the 36 streams surveyed in the Taranaki ring plain water quality survey (TCC 1984). Critical limiting nutrient levels for DRP to prevent proliferation of some common filamentous green algae appear to be as low as g nrr 3 (Wuhrmann & Eichenberger 1975; Freeman 1986; Seeley 1986). Since the levels of DRP are very high in pond effluents, dilutions of > 1700-fold would be required to limit algal growths. Temporal variations of pond discharges may reduce stimulation of algal growth and the downstream extent of proliferations. Detailed studies on the effects on river communities have not been undertaken. The concentration of dissolved inorganic nitrogen (DIN) present may also stimulate algal growths. Based on optimal algal growth at a DIN:DRP ratio of 7.8:1 (by weight) (Rhee & Gotham 1980) and the available data on phosphorus limitation, DIN concentrations of g nr 3 might be expected to limit growth. Field observations also indicate nitrogen limitation at O.O5O-O.O55 g DIN nr 3 (Stockner & Shortreed 1978; Grimm & Fisher 1986). A dilution of over 2700-fold in DIN-free water would be required to limit algal growths. Although further work on New Zealand rivers is required to identify limiting nutrients, the high concentrations of both DRP and DIN in dairy pond effluents suggest that algal proliferations in receiving streams should be anticipated except where high dilutions are available. The ammonia concentrations in pond effluents require > 250-fold dilution to reduce concentrations below toxic levels for sensitive fish populations. At the 95 percentile concentration for the pond with the highest levels (397 g nr 3, pond S4 in Fig. 2F), a dilution of > 500-fold would be required to meet ammonia criteria (USEPA 1985; 4-day average= 0.77 g nr 3 at ph 8, and temperature 20 C for salmonid waters). The toxicity of ammonia increases greatly with increasing ph. Proliferations of aquatic plants will result in more frequent high ph excursions (e.g., Freeman & McFarlane 1982; Quinn & Gilliland 1989) resulting in increased toxicity. Nitrification reduces the ammonia concentration, and hence decreases the toxicrisk, as well as consuming oxygen as discussed above. Findings in this study suggest that dairy ponds represent a significant toxic risk to fish populations, particularly when multiple discharges occur. Bacterial contamination of receiving waters may severelyrestrictrecreational bathing and water supply uses. Based on USEPA (1985) and New Zealand (Water and Soil Conservation Act 1967; Fourth schedule) criteria for recreational bathing, dilutions of > 2700-fold are necessary to support this use with 95% confidence that bacterial concentrations

13 Hickey et al. Dairy shed oxidation ponds 581 immediately downstream will not exceed the criteria (Table 6). Disease risk to human users from dairy ponds may be low given that these coliforms are exclusively of animal origin; however they mask contamination from human faecal sources present in the receiving water. Furthermore, Vanderholm (1984) lists a number of disease organisms which may be transmitted from animals to man including viral, fungal, and bacterial infections. The diseases spread through faecal contamination are various and include salmonella, streptococcus and staphylococcus infections, whereas leptospirosis is spread through urine from infected animals. The degree of survival of these disease organisms in oxidation pond systems is unknown; however, a human disease risk does appear to be associated with dairy pond wastes. Monitoring To determine likely effects on the receiving water it is necessary both to measure appropriate variables which may be used to predict impacts, and to have information on the concentration and likely variability of these constituents. Given the high variability both within and between ponds, and the number of constituents which may significantly impact the receiving waters, considerable simplification of a sampling programme becomes necessary. The relationship between the median BOD and median SS of ponds was good and suggests that the more readily measured S S may be used to provide an indication of pond performance (Fig. 3B). Faecal coliforms were a high proportion of the total coliforrn organisms in the dairy ponds and would be a preferred index of bacterial character. Conductivity correlated well with TP concentrations and appears, therefore, to provide a simple index for the concentration of this nutrient (Fig. 3A). Thus a minimum pond monitoring programme could involve measurement of conductivity and SS (as indices respectively for TP and BOD) as well as faecal coliforms and ammonia, because of their importance for public health and toxicity to stream organisms. Possibly a further simplification could be made if SS measurement could be replaced by field turbidity or clarity measurement as is suggested by our recent work on domestic sewage ponds (Hickey et al. 1989). The cumulative frequency distributions given in Fig. 1 provide an indication of the overall probability of a particular variable exceeding a given level. In addition to their utility for guiding water right decisions, such information may be used to design monitoringprogrammes and laboratory analytical requirements (e.g., BOD dilutions) or to determine the probability of occurrence of a single sampling event relative to this datasel Guidance as to the number of samples required is given in Table 3. Implications for pond effluent discharge management The high dilutions required to prevent undesirable receiving water impacts, particularly ammonia toxicity to fish (Table 6), suggests that it is unacceptable to use "general authorisations" that automatically permit the discharge to rivers of the effluents from dairy ponds that meet MAF specifications. In a recent survey, 36% of the 11 Regional Water Boards canvassed useof such general authorisations (Quinn & Hickey 1987). Where multiple discharges occur to sensitive rivers (e.g., where dilutions are low andph high), this is expected to cause breaches of water classification standards because of toxicity and public health effects. For example, levels of ammonia are so high that for a pond effluent diluted 100-fold by receiving water, concentrations that are potentially toxic to salmonids would be discharged c. 50% of the time, with 250- fold dilution required to limit toxic excursions to <5% of the time (further refinement of this example would of course be necessary for processing a particular discharge application because of ph and temperature considerations). The Planning Tribunals have ruled (e.g., NZTPA 1985) that effluents with S S concentrations in excess of 100 g nr 3 cannot be considered to be "substantially free from suspended solids", as required for their discharge to classified waters (approximately half of New Zealand's inland waters). The SS concentration in dairy ponds exceeds this level 80% of the time (Fig. IE) which presents a legal anomaly. Dairy shed ponds are generally performing poorly in comparison with the effluent concentrations of constituents from domestic sewage oxidation ponds (Hickey et al. 1989). Dairy ponds had higher median concentrations of BOD (by 4-fold), SS (by 5-fold), and ammonia (by 11-fold) than domestic sewage ponds. Pond DO levels were appreciably lower (by 3-fold) than in sewage ponds, suggesting less favourable conditions for biochemical oxidation processes and subsequent effluent load reduction. This comparison suggests that the general design criteria applied to dairy shed ponds may be inadequate and that some revision is desirable.

14 582 New Zealand Journal of Marine and Freshwater Research, 1989, Vol. 23 ACKNOWLEDGMENTS We are grateful to Ms G. F. Croker for data entry and preliminary statistical analysis; and to Mr B. W. Gilliland and Mr L. R. McKenzie of the Manawatu and Southland Catchment Boards and others who collected, analysed, and collated the chemical and microbiological data. We are also grateful for the comments received on an early manuscript draft from: D. B. Bottcher, B. W. Gilliland, E. Goldberg, E. T. Grogan, R. H. S. McColl, L. R. McKenzie, W. A. Taylor, R. Zuur, and two anonymous referees. REFERENCES APHA/AWWA/WPCF 1981: Standard methods for the examination of water and wastewater. 15th ed. American Public Health Association, Washington D.C. 1985: Standard methods for the examination of water and wastewater. 16th ed. American Public Health Association, Washington D.C. Cooper, A. B. 1984: Activities of benthic nitrifiers in streams and their role in oxygen consumption. Microbial ecology 10: : Developing management guidelines for river nitrogenous oxygen demand. Journal of the Water PollutU>nControlFederation58:&45-&52. Cosser, P. R. 1982: Lagoon algae and the BOD test. Effluent and water treatment journal, Sep: Dakers, A. J.; Drysdale, A. B.; Smith, K. A.; Vanderhohn, D. H. 1984: Anaerobic treatment. In: Vanderholm, D. H. ed., Agricultural Waste Manual, New Zealand Agricultural Engineering Institute, Lincoln College, New Zealand. Dakers, A. J.; Painter, D. J. 1983: Livestock waste management in New Zealand. In: P.N. McFarlane ed., Waste stabilisation ponds. Proceedings of the 15th New Zealand Biotechnology Conference. Biotechnology Dept., Massey University, Palmerston North, New Zealand. Dawson, R. H.; Robinson, W. N. 1984: Submerged macrophytes and the hydraulic roughness of a lowland chalk stream. Verhandlungen der internationalen Vereinigungfiir theoretische und angewandte Limnologie 22: Drysdale, A. B.; Painter, D. J. 1983: Lagoon-storageirrigation dairy waste system. Pp In: McFarlane, P. N. ed.. Waste stabilisation ponds. Proceedings of the 15th New Zealand Biotechnology Conference. Biotechnology Dept., Massey University, Palmerston North, New Zealand. Elliott, J. M. 1977: Statistical analysis of samples of benthic invertebrates. Freshwater Biological Association, Windamere, England. Scientific publication 25: 129. Finkelstein, J. 1973: Survey of New Zealand tank evaporation. Journal of hydrology (N.Z.) 12(2): Freeman, M.C. 1986: The roleof nitrogen and phosphorus in the development oicladophora glomerata (L.) Kutzing in the Manawatu River, New Zealand. Hydrobiologia 131: Freeman, M. C; McFarlane, P. N. 1982: Algae in the Manawatu River. Soil and water 18: Gaudy, A. F.; Gaudy, E. T. 1981: Microbiology for environmental scientists and engineers. Tokyo, McGraw-Hill. 736 p. Gloyna, E. F.; Tischler, L. F. 1980: Recommendations for regulatory modifications: the use of waste stabilization pond systems. Journal of the Water Pollution Control Federation 53: Grimm, N. B.; Fisher, S. G. 1986: Nitrogen limitation in a Sonoran Desert stream. Journal of the North American Benthological Society 5: Hickey, C. W. 1988: Benthic chamber for use in rivers: testing against oxygen mass balances. Journal of environmental engineering, ASCE114: Hickey, C. W.; Rutherford, J. C. 1986: Agricultural point source discharges and their effects on rivers. New Zealand agricultural science 20: King, D. L.; Tolmsoff, A. J.; Atherton, M. J. 1970: Effect of lagoon effluent on a receiving stream. In: McKinney, R.E. ed., Second International Symposium on Waste Treatment Lagoons, University of Kansas. MAF 1983: Effluent pond construction. Ministry of Agriculture and Fisheries, Aglink, FPP 291. McFarlane, P. N. ed., 1983: Waste stabilization ponds. Proceedings of the 15th New Zealand Biotechnology Conference. Biotechnology Dept., Massey University, Palmerston North, New Zealand. New Zealand Meteorological Service 1984: Rainfall Normals for New Zealand 1951 to New Zealand Meteorological service miscellaneous publication 185. New Zealand Microbiological Society 1976: Report of recommendations of the New Zealand Microbiological Society's Committee oncoliform bactena.l916.newzealandjournalofsciencel9: Nichol, S. E.; Basher, R. E. 1986: Analysis of three year's measurements of erythemal ultraviolet radiation at Invercargill, New Zealand. New Zealand Meteorological Service scientific report p. NZTPA 1985: New Zealand Paper Mills Limited v. Southland Regional Water Board. New Zealand town and country appeals 11: Quinn, J. M.; Gilliland, B. W. 1989: The Manawatu River clean-up has it worked? Transactions of the InstituteofProfessionalEngineersofNewZealand 16(1):