Software for Effluent Recycling through Intensive Silviculture

By Dr. Chris S. Papadopol
September 2000

The Author is Lead Scientist, Ecology of Intensive Plantations, with the Ontario Forest Research Institute in Sault Ste. Marie, Ontario, Canada. OFRI develops new scientific knowledge to support the sustainable management of forests, wildlife habitats and biodiversity.   → See also:

Recycling of municipal wastewater pose increasingly serious challenges to communities. In some situations, recycling is possible and desirable by means of specialized fast growing plantations, that require both water and nutrients and provide an aerated soil environment that fosters decomposition of the organic load. At the Ontario Forest Research Institute (OFRI) in Sault Ste. Marie, Canada, a software package has been developed for operating such a recycling system on the basis of weather and soil conditions. It allows daily irrigation with effluent of a hybrid poplar plantation with an amount of effluent corresponding to atmospheric demand for evapotranspiration in a particular day, thus avoiding the risk of deep infiltration.

What is plantation recycling ?

Recycling municipal effluent through a tree plantation is an ecologically sound and increasingly attractive approach to wastewater management. The trees absorb the water for transpiration and metabolize the chemical load through timber biomass production. As more people become aware of the advantages, especially the lower cost when compared to activated sludge process, their number is bound to increase. Because they operate outside the food chain and require little energy, they may be the most suitable for municipal effluents and small communities, resorts, etc. Such recycling could be extended to other naturally decomposable effluents from concentrated animal operations, possibly with a heavy load of coliforms and antibiotics residuals. However, the safeguards necessary for these open-environment systems must become stricter.

A plantation recycling system is composed of a forest plantation, a storage lagoon and a distribution network (pumps, pipe and sprinklers). It can be located at low cost on marginal agricultural land. Its driving force is the demand for transpiration, which depends on weather, the hotter and dryer the weather, the more water the trees take up.

This software is a flexible instrument that allows the user of an effluent spray field, planted with trees, to be load it with an amount of effluent corresponding to the demand for evapotranspiration, determined at short time intervals. This allows also for the integration of rains in the irrigation schedule. This way the administered effluent causes only superficial variations of soil moisture, and deep infiltration of unprocessed effluent is prevented.

Why was the software developed ?

Recycling municipal waste water through tree plantations is a developing science. Only a few applications exist around the world, with various design approaches and even differing objectives. In general, they are operated on the basis of a flat irrigation rate, with no regard for the differences among days in terms of ETp amount. In Canada, under a sub-humid climate, strict environmental regulations aim to prevent groundwater contamination through irrigation with effluent. Therefore, recycling of effluent should be practiced only during the dry spells between rains. When conditions are favorable, sunny and/or windy weather, the recycling system has to be loaded up to its full capacity. Allowing for automation of irrigation automation activity, this software attempts to make full use of the ETp demand.

How was the software developed and how does it work ?

A hybrid poplar plantation was used as the recycling engine, mostly because it develops quickly large foliage that stays on trees throughout the growing season. Due to advection, greatly enhanced by air circulation in a well-spaced plantation, such a recycling system can transpire several times the «grass standard» potential evapotranspiration (ETp).

This program has interactive graphics for the visualization of some key parameters. Thus, Figure 1 presents the image of weather parameters during a sunny day, while Figure 2 displays the saturated and actual vapor pressure, as well as a rain event (histogram). The difference between these curves represents the moisture saturation deficit, or the «space» in the atmosphere in which additional moisture can be received. It disappears as soon as the rain begins.

The output of the program consist of archive files with watering and timing signals for the pump controller. Watering frequency is exemplified in Figure 3 and Figure 4, for the rates 20 and 40 m³/ha, corresponding to very permeable and normal soils, respectively.

If the soil moisture was also being monitored, the program SOIL downloads these data hourly, archives the moisture by depths and calculates the flux of water through the bottom of root zone. Depending on its value, the pump may receive a stopping signal, delaying the irrigation activity for 1 to n cycles, until the flux is again zero or lower than a pre-established threshold.

Functioning of plantation

We started this research project in 1983 at Seneca College, in King City, Ontario, using secondary treated effluent, sprayed during growing season over a 22 ha of hybrid poplar and willow plantation. Aside of regular soil maintenance in the first 2 years, the plantation needed pruning in the fifth year to allow for better air penetration. The effluent was distributed through underground plastic pipes and risers with high-pressure sprinklers with a «throw» radius of 36 m. The operation is still functional.

Data collected allowed us to define the new concept of environmentally sound effluent recycling combined with intensive silviculture, distinct from other applications in the following respects:

What is the future of recycling plantations ?

Plantation recycling can be an efficient eco-technology. It is based on sound environmental principles, is cheap, profitable, can be automated and processes the effluent outside the food chain. It is especially promising for poorer areas of the globe, which have increased risks of epidemics. This software has the potential to help ensure plantation recycling is conducted safely and efficiently. By using tree plantation as the recycling engine, such systems have a greater ETp, owing to a better connectedness with the atmosphere than agricultural crops. In turn, this means that smaller land area is necessary for recycling. Where conditions permit, this forest eco-technology should be adapted also for wastewater resulting from concentrated animal operations.

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