A novel separation system removes solids from pig effluent more effectively than other systems in common use

About 40% of Australia’s piggeries use anaerobic lagoons (ponds) to manage liquid waste, but associated maintenance and infrastructure costs can be significant. Also, methane from treatment ponds accounts for as much as 60–70% of greenhouse gases emitted across the Australian pork supply chain (Wiedemann et al. 2009). Lastly, ponds can be a significant odour source. For these reasons, there is interest in pondless effluent management systems that can recover manure as solids prior to substantial fermentation, thereby producing a solid cake for co-composting and a low-strength liquid waste (filtrate) for treatment in smaller less costly ponds or for direct recycling as flush water, thus creating a closed-loop system. The aim of this study was to evaluate a potentially pondless system consisting of a novel de-watering/filtering system, the Z-Filter (Z-Filter Pty Ltd, WA), on farm at a commercial pig shed housing 1,200 pigs aged 10 to 22 weeks.

For each filtration test run, an entire flush from any one of four flush lanes was collected in a 10 kL holding tank, to which 0.8 to 1.0 L of coagulant (Floquat FL 2949, SNF-Australia Ltd) was added while mixing. The flush manure was pumped from the holding tank, through a static mixer where a flocculant solution (a 0.5% solution; FlopamTM, SNF-Australia) was added at 38–45 mL/L, after which it passed through a floccule-maturator to grow floccules and then onto the Z-Filter. Filtrate was pumped into another holding tank before being recycled back to a flush tank for a following day’s flush, thus creating a closed loop. Flushing frequency varied from 2–3 times/week at the start of the pig batch to daily at the end. The Z-Filter works continuously with a fabric filter called a ‘sock’, which follows a triangular path closing it into a tube containing slurry, pressing it with rollers to remove water through its porous sock and then re-opening it to discharge dewatered solids. Eleven samples, each of flush manure (from the holding tank), filtrate and separated solids were collected over 11 weeks representing the four flush-lanes and the pig growth batch. These samples were collected from 20 L containers holding aggregates of 15 sub-samples, which were stirred/mixed to ensure homogeneity. All samples were stored at –20°C and air-freighted frozen on dry ice to Brisbane for analysis. Upon receipt (still frozen) the samples were further stored at –20°C prior to analysis. Samples were analysed (Gopalan et al. 2013) for total solids (TS), volatile solids (VS), volatile fatty acids (VFA, by gas chromatography), phosphate, oxidised nitrogen and ammonium nitrogen (ammonia N, by flow injection analysis), and total Kjeldahl nitrogen (TKN) and phosphorous (Total P). Minerals were analysed by ICP-OES after nitric acid digestion (Tait et al. 2009).

Removal extents achieved by the Z-Filter were higher than for other similar solids separation systems in common use. Despite significant variation in TS of the flush manure over the trial period (flush manure into the Z-filter contained 1.3–2.4 wet mass % TS), the Z-filter sustained removal extents at around 58%. Other removal extents averaged 73% for VS, 35% for TKN and 50% for total P. However, the Z-Filter (as with other mechanical systems) was unable to remove colloidal and dissolved compounds, with removal extents for ammonia nitrogen (14%), potassium (10%) and VFA (16%) being low. Therefore, further treatment would be required for the filtrate of the Z-filter in onsite ponds, albeit with estimated 60% smaller pond sizes. The separated solids had an average dry matter content (TS) of 22 wet mass %, and were stackable with minimal seepage and easily transportable.

The present Z-filter trial produced a solid cake suitable for co-composting and a low-strength liquid waste (filtrate) for treatment in smaller/less costly onsite ponds. However, filtrate recycled as flush water over extended periods would require some further treatment to remove soluble compounds. Preliminary economic modelling for a 2,000 sow farrow-to-finish conventional piggery estimated capital and operating costs for a Z-Filter to be around $50 and $132/t TS processed, or $0.04 and $0.12 per kg of dressed finisher weight sold/y for low and high flush volumes, respectively. These costs currently are similar to conventional pond systems, but opportunity exists to reduce chemical costs of the Z-filter. Further work is required to quantify other potential benefits of pondless systems, such as enhanced use of nutrients, reduced water use, reduced odour, and site constraints that may limit the use of conventional pond systems.