Wastewater-fertigated Short Rotation Coppice (wfSRC) Systems for domestic wastewater treatment

The initial step for the design of a wfSRC system is to determine and quantify the total volume of wastewater to be treated and the climatic and soil conditions of the location. Including, for instance, parameters such as the average annual precipitation, potential evotranspiration, infiltration rates and granulometry. Generally, the system is constructed with a width of 8 metres (m,) minimum depth of 1.5 m, leaving 60 centimetres (cm) between the plants, 75 cm between the rows and with slopes on the sides. However, the required dimensions are to be defined according to the identified parameters (LANGERGRABER et al. 2019).

The wastewater (WW) must be pre-treated by means of a sedimentation tank and a grit removal, before being collected in an equalization tank from which the wfSRC bed is loaded. The pre-treated wastewater is distributed within the system by a level-controlled pump, the distribution pipes can be located under or above the ground. A drainage pipe is placed in the bottom of the bed and is used to empty water from the bed, if there is salt accumulation after some years of operation. Additionally, the field needs to be enclosed using a small dam levee to retain the WW on the respective area that is used as wfSRC and for the WW to flow into the low- lying land.

Fig. 1 depicts a typical wfSRC system with underground pipelines:

Fig. 1: Schematic of a zero-discharge willow system. Source: LANGERGRABER et al.  (2019)

Among the most suitable species for a wfSRC systems, are Eucalyptus citriodora, Alix tetraspermo, Bambusa bulgaris and several willow and poplar species. The harvesting periods for this species are usually once every 2 - 3 years, where between one half and one third of the crops should be harvested to keep the plants in a young and healthy state with high transpiration rates.

The following aspects need to be considered when deciding if this technology is approporiate for a particular site location or fits the existing context in general:

• The location of the field requires large surface and intermediary zones that protect the system against flooding in case of heavy rainfall. Hence, the system is genereally suitable for rural areas. wfSRC systems are not appropriate for large cities with space limitations.
• In general, wfSRC systems are not appropriate for applying highly polluted industrial WW that varies in composition and characteristics.
• The existing soil, climate conditions and water parameters have a direct impact on the productivity of the system. The plant species used usually do not demand very high quality agricultural soils. Soils with pH 5 - 7.5 will produce satisfactory growth, although research suggest that there is plant material that is tolerant to pH outside this range (CASLIN et al. 2015). In dry areas, light sandy soils will probably have a problem in terms of water availability and therefore may be avoided. The same is valid for shallows soils which will provide low yields. The cultivation on flood lands or sensitive wetland areas needs to be carefully assessed since the cultivation with heavy machinery can be challenging and can have a negative impact on wet soils, such as soil compaction (DIMITRIOU and RUTZ 2015).
• The layout of the wfSRC must be designed to optimise crop densities and yield, to fit into the surrounding landscape and to allow easier operation and access of the machinery involved in the setting up and harvesting processes. Also, in order to keep the system economically, the distance of the wfSRC and WW source should be short (energy for pumping). Further information regarding the layout of wfSRCs can be found in chapter two of the Sustainable Short Rotation Coppice Handbook (DIMITRIOU and RUTZ 2015).

The establishment of the system requires low energy and earth work. However, preparation of the soil, such as leveling, installation of pumps and pipes, among others, are necessary. Since only basic construction materials are needed they can in most locations be easily sourced locally. The plant species are easily accessible, as the species used grow in many parts of South and Southeastern Asia.

In general, the operation and maintenance of the system do not require complex, high demanding processes, nor intense labour. However, on a regular basis, the operation of the system includes daily standard activities such as opening and closing the valves, running the pumps, besides crop harvesting that is to be executed every 1 - 3 years. These activities can be carried out by a local operator that received basic training on-site. Additionally, qualified personnel is required for the maintenance of the equipment which is executed every year (or on-demand), and the standard agricultural practices, such as planting, conservation, plantation monitoring and weed control which needs to be performed twice per month during the first year and at least once per month after the second year.

The monitoring campaigns as well as their frequency, are subjected to the legislation requirements stated in each country. Ideally, for general performance monitoring, operational parameters such as Chemical Oxygen Demand (COD), Phosphorus (P), Potassium (K), Total Organic Carbon (TOC), the the Biochemical Oxygen Demand (BOD5) and pathogens along with groundwater quality should be monitored regularly, varying from a weekly to a bi-monthly basis, depending on the location and the complexity of the system.

The experiences in India with natural based systems, and wfSRC specifically, are limited. However, wfSRC systems are expected to be key to improve the situation of water resources and forest ecosystems in India. They have the potential to offer local communities the opportunity to produce their own clean, renewable resources to cover their energy needs, while contributing to reduce pollution and increase the positive environmental impacts and lower negative health impacts of formerly untreated wastewater streams.

An example for the few experiences made in India, is the NAWATECH project, which among other activities, piloted a wfSRC at the Ordenance Factory Ambajhari (OFAJ) in Nagpur. The pilot line consisted of a French reed bed followed by a wfSRC. The local species selected for this pilot treatment line were Melia Dubia and Bambusa bambos (ECOSAN CLUB 2016). However, there is no information on the actual performance of this pilot available.

As described by SINGH et al. (2011), another wfSRC was implemented in Jodhpur (North West India), where the species Acacia nilotica and Eucalyptus camaldulensis were irrigated with municipal wastewater. A second study was conducted in Palwal, a district located 70 kilometres from New Delhi, in which the performance of the species Tereticornis eucalyptus, Melia Azedarach, Ailanthus excels were evaluated after being planted and irrigated with secondary treated wastewater (TOKY et al. 2011). Relevant results were obtained from the projects and valuable data regarding a.o. biomass yield per specie or the nutrient uptake rate.

wfSRCs are known and used in many parts of the world. Especially in Northern Europe and the US commercial systems are in place as described by Haenel et al. (2022). Taking into account the general potential and the wastewater situation, a large-scale implementation in rural areas in South-East Asia but also in Latin America are expected to have very promising potential.

As the wfSRC systems have their origin northern European countries, most of the successful case studies are found for this region. One successful example is the “Enköping model” (BÖRJESSON and BERNDES 2006), where the establishment of a wfSRC in a city located in central Sweden was considered as necessary to complement the regional biomass demand.

Worldwide, for instance in Denmark, Poland, Estonia, Germany, Canada, Australia, Brazil, aspects around wfSRC’s soil and water purification, biomass and bioenergy production, environmental purposes, economic analyses, as well as nutrients dynamics, were studied (GUIDI et al. 2015, DE OLIVEIRA et al. 2014, HOLM and HEINSOO 2013, LIESEBACH et al. 1999, MORTENSEN et al. 1998, HAENEL et.al. 2022).

wfSRC systems have the potential to address in a cost efficient way, two of the most concerning environmental problems nowadays, water pollution and climate change by treating the WW in a sustainable way while producing renewable energy sources. Consequently, the investigation and experience obtained from the different areas will allow practitioners, academics and researchers to gain a better understanding of the systems, and to identify potential challenges and how to successfully overcome them. Besides, boosting the implementation of more technologies in many countries, which benefit both, the communities and the environment.