Impact of aeration on the removal of organic matter and nitrogen compounds in constructed wetlands treating the liquid fraction of piggery manure

The increasing demand for sustainable, robust and cost-efficient wastewater treatment techniques strengthen the implementation of constructed wetlands (CWs) in the agricultural sector. In countries like Belgium (Flanders), the compliance of strict water quality standards and surface area requirements have hindered considerably their application. New wetland designs such as aerated CWs, could help to overcome these challenges. This study evaluated the capacity of artificially aerated mesocosm systems to decrease chemical oxygen demand (COD) concentrations below the 125 mgO(2)/L limit imposed on installations treating animal manure. The treatment of this high-strength wastewater has been slightly studied via aerated CWs. A three-stage experiment investigated the effect of constant, intermittent and non-aeration regimes on: ammonium volatilisation, the evolution of organic and nitrogen compounds concentrations, and denitrification. The results were assessed through a mixed modelling procedure using SAS 9.4 software. A COD removal between 65% and 58% in constantly and intermittent aerated systems, versus 27% COD removal in the non-aerated system indicated the effectiveness of aeration. However, a dissimilarity was encountered in the removal of nitrogen compounds, resulting in an 82% decrease of nitrate concentrations in the non-aerated system, versus 0.5% and 11% in the aerated ones. Based on the results, this experimental set-up adjusted to field operational conditions can prove that aerated CWs can treat the liquid fraction of piggery manure

Assessing the integration of wetlands along small European waterways to address diffuse nitrate pollution

Nitrate concentrations in numerous European fresh watercourses have decreased due to end-of-pipe measures towards manure and fertilization management, but fail to meet the environmental objectives. The implementation of complementary measures to attenuate diffuse nitrate pollution in densely populated regions characterised by limited available area has been barely studied. To tackle this issue, this study evaluates the feasibility of integrating Constructed Wetlands (CWs) along waterways as a promising tool to facilitate compliance with the nitrate regulations. The aim is to calculate the required area of land alongside a specific watercourse to integrate CWs to reduce nitrate concentrations consistently below the 11.3 and 5.65 mgNO(3)-N/L levels, according to the Nitrates Directive and the Flemish Environmental Regulations. Nitrate-nitrogen removal efficiencies achieved at case study CWs were compared and validated with reported values to estimate the needed wetland areas. In addition, the removal efficiencies and areas needed to meet the standards were calculated via the kinetic model by Kadlec and Knight. The predicted areas by both methods indicated that CWs of 1.4-3.4 ha could be implemented in certain regions, such as Flanders (Belgium), with restricted available land. To conclude, three designs for ICWs (Integrated Constructed Wetlands) are proposed and evaluated, assessing the feasibility of their implementation

Assessing the ecological relevance of organic discharge limits for constructed wetlands by means of a model-based analysis

Polder watercourses within agricultural areas are affected by high chemical oxygen demand (COD) and biological oxygen demand (BOD5) concentrations, due to intensive farming activities and runoff. Practical cases have shown that constructed wetlands (CWs) are eco-friendly and cost-effective treatment systems which can reduce high levels of organic and nutrient pollution from agricultural discharges. However, accumulated recalcitrant organic matter, originated by in-situ sources or elements of CWs (i.e., plants or microbial detritus), limits the fulfilment of current COD discharge threshold. Thus, to evaluate its relevance regarding rivers ecosystem health preservation, we analysed the response of bio-indicators, the Multimetric Macroinvertebrate Index Flanders (MMIF) and the occurrence of organic pollution sensitive taxa towards organic pollutants. For this purpose, statistical models were developed based on collected data in polder watercourses and CWs located in Flanders (Belgium). Results showed that, given the correlation between COD and BOD5, both parameters can be used to indicate the ecological and water quality conditions. However, the variability of the MMIF and the occurrence of sensitive species are explained better by BOD5, which captures a major part of their common effect. Whereas, recalcitrant COD and the interaction among other physico-chemical variables indicate a minor variability on the bio-indicators. Based on these outcomes we suggest a critical re-evaluation of current COD thresholds and moreover, consider other emerging technologies determining organic pollution levels, since this could support the feasibility of the implementation of CWs to tackle agricultural pollution

Environmental impact assessment (EIA) of effluents from constructed wetlands on water quality of receiving watercourses

The amount of animal manure produced in Flanders—Belgium by intensive animal farming generates a surplus that needs to be treated in order to achieve quality objectives set by the Nitrates Directive (91/676/ EEC) and the European Water Framework Directive (2000/60/EU). After the physical separation and biological nitrification/denitrification processes, the liquid fraction of manure can be cost-efficiently and effectively treated by constructed wetlands (CWs). However, current discharge criteria limits do not evaluate whether nutrient loads from specific point sources (such as CWs) affect the water quality of their receiving waterway. Thus, we investigated whether a site-specific analysis, based on local environmental conditions, would yield more relevant discharge thresholds. In the present study, a standardized framework was developed for environmental impact assessment (EIA) of effluents from CWs on the water quality of receiving watercourses. This framework was tested as a case study on a manure treatment installation located in Langemark—Belgium. The effect of different impact scenarios on water quality and flow of the effluent and the receiving waterway was studied. Standardized EIA guidelines and sensitivity analyses were applied to determine the expected impacts of total nitrogen (TN), total phosphorous (P), chlorides (Cl−) and sulphates (SO42−) on the receiving watercourse. From this study, we concluded that the methodology currently applied requires adaptation when assessing the discharge from wetlands as current estimations of impact are overly conservative when compared with actual impact. In addition, results showed that expected impact might be mitigated by differentiating discharge limits between dry and wet periods

Agronomic and environmental performance of Lemna minor cultivated on agricultural wastewater streams : a practical approach

This study investigated the potential of Lemna minor to valorise agricultural wastewater in protein-rich feed material in order to meet the growing demand for animal feed protein and reduce the excess of nutrients in certain European regions. For this purpose, three pilot-scale systems were monitored for 175 days under outdoor conditions in Flanders. The systems were fed with the effluent of aquaculture (pikeperch production—PP), a mixture of diluted pig manure wastewater (PM), and a synthetic medium (SM). PM showed the highest productivity (6.1 ± 2.5 g DW m−2 d−1) and N uptake (327 ± 107 mg N m−2 d−1). PP yielded a similar productivity and both wastewaters resulted in higher productivities than SM. Furthermore, all media showed similar P uptake rates (65–70 P m−2 d−1). Finally, duckweed had a beneficial amino acid composition for humans (essential amino acid index = 1.1), broilers and pigs. This study also showed that the growing medium had more influence on the productivity of duckweed than on its amino acid composition or protein content, with the latter being only slightly affected by the different media studied. Overall, these results demonstrate that duckweed can effectively remove nutrients from agriculture wastewaters while producing quality protein

Evaluation of a new approach for swine wastewater valorisation and treatment : a combined system of ammonium recovery and aerated constructed wetland

Nitrate Vulnerable Zones (NVZs) are faced with a surplus of animal manure due to intensive livestock produc-tion, and the high use of mineral nitrogen (N) fertilisers in crop production. Recovery of N from animal manure to replace synthetic mineral fertilisers is considered a key strategy to close the N loop for more sustainable agriculture and to meet strict legal frameworks. In this study, N recovery from swine wastewater by an ammonia (NH3) stripping process followed by purification via an aerated constructed wetland (ACW) was proposed as an alternative approach to conventional systems based on biological nitrification-denitrification (NDN) treatment. The performance of the NH3 stripping pilot as well as the ACW was monitored in 2019-2020 over three periods, to evaluate the quality of recovered ammonium nitrate (AN) solution and the effluent of the ACW. Results showed that the NH3 stripping unit recovered 21% of total-N (32% of mineral-N) in the form of AN solution. This could be used as a mineral fertiliser according to the criteria of the European Fertilising Products Regulation 2019/1009 and the technical proposal of manure-derived RENURE (REcovered Nitrogen from manURE) prod-ucts by the European Joint Research Centre. As a RENURE product, AN solution would reach an end-of-manure status and could be used as a synthetic N fertiliser replacement. The tested ACW achieved a high removal ef-ficiency with respect to suspended solids (96%), biological oxygen demand (96%), chemical oxygen demand (90%), total-N (80%), and total phosphorus (97%). The quality of ACW effluent was comparable to that of NDN treatment. Though the overall cost of the proposed pilot-scale process consisting of NH3 stripping (5.1 euro t-1) and ACW (12 euro t-1) was calculated slightly higher than conventional NDN treatment (16 euro t-1), it is foreseen to outcompete at a higher loading rate (over 45 m3 ha-1 d-1). Furthermore, post-purification will be needed for the ACW effluent to meet the requirements for discharge to surface water