Dairy Shed Effluent - 3.0 A review of two pond system design and performance

3.1 BOD5 and Suspended Solids
3.2 Nitrogen and Phosphorus in DSW Ponds Effluent
3.3 Pathogen Indicators in Two-Pond System Effluent
4.1 Anaerobic Ponds
4.2 Aerobic Ponds

3.0 A review of two pond system design and performance

3.1 BOD₅and Suspended Solids

Two-stage pond systems - an anaerobic pond followed by a facultative or aerobic pond - have long been recognised as the most cost-effective pond systems for treating higher strength (compared to domestic sewage) wastewater for BOD₅reduction. Conceptually two-stage pond systems were considered ideal for treating dairy shed wastewater when the focus was on BOD and SS reduction.

Although design guidelines were available, and many ponds were installed in the late seventies and early eighties, the appearance of Agricultural Waste Manual of the NZIAE in 1984 - providing comprehensive design and installation guidelines for two-pond systems for dairy shed effluent - was a spur to an increased rate of ponds installation. Guidelines published subsequently by MAF (Aglink) and others adhered to the loading and engineering recommendations set out in the Agricultural Waste Manual (AWM).

The design guidelines for anaerobic ponds were drawn from American experience with anaerobic ponds for farm manure-based wastewaters. Design criteria are based on the pond providing the following functions:

solids separation (settling),

solids digestion (solubilization of biodegradable organic material),

anaerobic oxidation of dissolved organic material (reduction of the BOD in the effluent),

storage of digested solids residues and other non-degradable and slowly degradable solids,

passage out of the pond of partially treated wastewater

In these functions, an anaerobic pond must be seen as functionally analogous to a septic tank rather than an anaerobic digester.

The first four items in the list above occur with variable efficiency depending on the loading rate, temperature, engineering details of the pond, and maintenance of the pond, particularly with respect to de-sludging and crust control.

The sizing of anaerobic ponds putatively includes solids storage volume such that after five to ten years of sludge build-up there will still be a liquid, settling zone of sufficient volume to maintain performance efficiency of the pond.

The AWM provides two tables (Tables 2.2 & 2.3) giving average and range values for waste production from dairy sheds and from cows. Common practice is to use the average values from these tables to calculate waste production for the purpose of sizing individual pond systems. The recommended loading rates for anaerobic ponds are 28, 24, and 20 g BOD₅/m³/day of pond volume for northern middle, and southern regions of NZ respectively.

It is assumed in the sizing guidelines that a correctly designed, installed, and maintained anaerobic pond achieves 70% removal of BOD₅from the effluent. This assumption is used in calculating the load to the second, "aerobic pond" and therefore the sizing of it. A loading rate of 8.4g BOD₅/ m²/day of pond surface is recommended for the second pond in two-pond systems, this rate being the same as is suggested for Facultative sewage oxidation ponds by the Ministry of Works & Development (1974).

The AWM suggests that a further removal of 80% or more of BOD₅could be expected over an aerobic lagoon (= facultative pond) treating anaerobically pre-treated DSW, giving overall removal in the order of 95% over the two ponds. From the text of the AWM and implied in the fact that a loading rate or 8.4 g BOD₅/m²/day was deemed appropriate for the second pond, it is apparent that the second pond was expected to function and perform the same as a facultative sewage oxidation pond.

At an average BOD₅concentration of 1,500 g/m³in DSW, 70% removal during treatment in an anaerobic pot reduces this to 450 g BOD_5/m^3.With the minimum expected removal of 80% by the "aerobic" second pond, giving 94% removal overall for the system, the maximum concentration of BOD, expected in the final effluent should be about 90 g/m³. A 30% loss of water by evaporation as the wastewater passes through the ponds would increase the concentration (but not the total mass) of BOD₅in the final effluent to about 128 g/m³. Conversely, infiltration of groundwater or stormwater drainage into the system will result in reduced concentration BOD₅in the effluent.

A report by Hickey et al (1989) examining the performance of DSW ponds in the Manawatu and Southland found that the average effluent BOD and suspended solids concentrations from eleven functional DSW two-pond systems was as expected (geometric mean BOD₅90g/m³and SS 227 g/m³). However, the variation on the average was large: an 8-fold difference between the 5 percentile and 95 percentile values about the mean in the BOD₅data.

In contrast, a recent survey of 19 functioning two-pond systems by the Waikato Regional Council Found geometric mean effluent BOD₅of 143 g/m³, with a range of 50 to 360 (unpublished data). Geometric mean BOD₅of the anaerobic ponds effluent was 228 g/m³, rather less than the 450 g/m³expected in the effluent from an anaerobic pond. More importantly the reduction of mean BOD₅ from 228 to 143 is passage through the "aerobic" ponds denotes a treatment efficiency of only 37% for the aerobic ponds - much less than the 80% more expected from these ponds. It would appear that anaerobic ponds may perform better than expected and aerobic ponds worse than expected.

In Northland, for five ponds systems repeatedly sampled overall seasons, the effluent BOD₅means were 113, 89, 203, 121, and 181 g BOD₅/m³(unpublished data).

3.2 Nitrogen and Phosphorus in DSW Ponds Effluent

Two-pond systems area technology primarily for reducing the BOD of wastewater. Although reduction of the and P content of the wastewater is not a design objective, about 85 % mass removal of total nitrogen and 70 mass removal of total phosphorus may be expected from a well-functioning two-pond system.

About 55 percent of the nitrogen and 35 percent of the phosphorus are removed from the effluent over the anaerobic pond. Most of the nitrogen removal and all of the phosphorus removal in an anaerobic pond is by sedimentation of these elements bound in particulate organic matter. Of the nitrogen that remains in the effluent, some is inorganic form in unsettled particulate and colloidal organic material, and some is as ammonia nitrogen, converted to this form during digestion and anaerobic oxidation of organic matter in the DSW. Some ammonia nitrogen is lost by volatilisation of un-ionised ammonia, with the greater part remaining in solution in ionised form in the effluent.

Removal mechanisms for nitrogen in passage through the second pond include sedimentation of particulates and ammonia volatilisation. (The sedimentation pathway includes nitrogen in algal and bacterial biomass.) The ammonia volatilisation kinetics for oxidation ponds described by Pano and Middlebrooks (1982) and Reed (1985) would suggest that volatilisation can account for the major part of nitrogen loss from effluent overt second pond.

Phosphorus removal in the second pond continues as sedimentation of particulate organic phosphorus.

3.3 Pathogen Indicators in Two-Pond System Effluent

Reduction of pathogen numbers in DSW over two-pond treatment occurs as attenuation (die-off) over time and is therefore related to mean hydraulic residence time (MHRT). Actual MHRT is usually less than the theoretical MHRT (system volume divided by daily flow, typically more than 100 days) because of variation in the effectiveness of wind mixing, and short-circuiting due to badly positioned inlet and outlet points.

Hickey et al (1989) report a geometric mean of 40,000 faecal coliforms per 100 ml for 72 samples taken from "aerobic" ponds in Manawatu and Southland. The 5 percentile and 95 percentile values around this mean are 200 and 540,000 respectively.