Integrated Slurry Management System with Constructed Wetlands

Horizontal subsurface flow constructed wetland for pig slurry treatment, preceded by physical pre-treatment. Inverted trapezoidal cross-section with impermeable liner and gravel-sand fill. Macrophyte roots enhance microbial degradation. Influent percolates without surface flooding. Hydraulic retention enables mechanical filtration, while plants transport oxygen to root zones. Reductions achieved for Total Nitrogen, Electrical Conductivity, Settleable Solids, CH₄ and NH₃ emissions.

OPERATION
The slurry is separated into liquid and solid fractions using mechanical separation equipment. If there is enough space, the separation may be further enhanced by a subsequent decantation step. Most of the phosphorus and a substantial portion of the nitrogen are retained in the solid fraction, which is a product that is easier to transport longer distances. This technique is fully applicable to the intensive pig rearing sector, as demonstrated at implemented sites.
The liquid fraction flows to a horizontal subsurface flow constructed wetland, characterized by an inverted trapezoidal cross-section lined with an impermeable double-layer plastic membrane to prevent filtration into the subsoil and avoid any leakage to the environment. Inside this passive biological treatment system for pig slurry, the influent enters by gravity across a required 5% slope, percolating subsurface through the main filling material—a gravel bed—without flooding the surface. The system contains macrophyte vegetation (e.g., Phragmites australis, Juncus, Typha, Apium nodiflorum) rooted in a sand layer that rests on the gravel.
As the liquid moves through the substrate, mechanical filtration occurs, and prolonged hydraulic retention times (3–5 days) allow effective purification. Simultaneously, wetland plants transport oxygen to submerged root zones, fostering microbial communities that biologically degrade pollutants and organic matter.

PERFORMANCE
Nitrogen (as NO₃⁻ and NH₄⁺) is absorbed by the plants, thus reducing ammonia (NH₃) losses to the atmosphere. Based on experimental data, the treated effluent contains N-NO₃⁻: 3.9–5.8 mg/L and N-NO₂⁻: <0.021 mg/L (raw slurry: 1.6–6.1 mg/L N-NO₃⁻; decanted slurry: 1.9–6.3 mg/L N-NO₃⁻). These anions arise from natural biological oxidation of NH₃, common in passive systems. Possible emissions of nitrous oxide (N₂O) are kept low with proper management of operational parameters. Suspended solids and Biological Oxygen Demand (BOD) are also reduced.
The treated effluent exits via a drainage pipe at the outlet, collected by gravity into a final collection pond, and can be reused in agriculture (e.g., through fertigation), since it contains remaining N and/or P. Specifically, it reduces total nitrogen load by up to 50%, complying with the 170 kg N/ha/year limit of the Nitrates Directive, and supplies 60–120 kg P₂O₅/ha and 350–550 kg K₂O/ha (seasonal variation), thus closing the nutrient cycle through industrial symbiosis between livestock farming and agriculture.
Energy consumption is very low but not zero: main equipment includes homogenization agitation (5.5–15 kW), a feed pump/phase separator (1.5 kW for 2–10 m³/h), and transfer pumps (0.75 kW each). Periodic harvesting of vegetation/biomass, which can be used as co-substrate for anaerobic digestion or as feedstock for composting, and removal of accumulated organic matter are done as part of proper management. Care should be taken to address possible cross-media effects.

Production data

The integrated treatment system is designed to process the entire slurry volume generated by the farm, with the decantation unit and number of constructed wetland cells sized accordingly to match actual production. The system achieves a 20% reduction in total volume from raw slurry to purified effluent. This volume reduction, combined with the significant decrease in N concentration, results in a substantial reduction in the agricultural land required for sustainable application. Under the Nitrates Directive (170 kg N/ha/year), the treated effluent requires up to 50% less land compared to raw slurry, transforming a challenging waste management issue into a manageable, circular solution that facilitates compliance with environmental regulations while recovering valuable water and nutrients for agricultural use.

Associated main production process(es) and product(s): The Main process is an integrated system to treat raw pig slurry through phase separation, decantation, and a horizontal subsurface flow constructed wetland. The products are a solid fraction and a purified effluent suitable for irrigation and its reuse.

Production data: 15 m3

Participant Companies