Ecological Engineering for Effluent Recycling through Intensive Silviculture

By Dr. Chris S. Papadopol
June 2004

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:

Contents

Introduction

During the past 3 - 4 decades, the fields of ecology and environmental science have developed rapidly, with the apparition of ecological engineering applications to waste water recycling as a normal process. However, such applications must be based on sound ecological knowledge and a thorough understanding of ecosystems and their response to environment. Ecological engineering is defined as environmental manipulations by society, using small amounts of supplementary energy to control systems in which the main energy drives are still coming from natural sources. In fact, this is a partnership with «organized» nature, perhaps defined as the use of technological means for ecosystem management, based on ecological understanding, to minimize the cost of basic functions and the harm done to environment. It uses the basic principles of science to design better applications for human society. However, when compared to other forms of engineering and technology, ecological engineering designs the human society with its natural environment, instead of trying to conquer it.

Recycling of human or animal wastes through an ecosystem, under natural conditions, is an ancient practice. The spread of liquid waste as fertilizer over agricultural crops is a habit originating with the ancient civilizations of the Middle East. However, large-scale, regulated, waste water application for irrigation only dates back to late last century, when some farm-like sewage operations were established first in India and China, and then in Europe and United States. More recently, some poorly conceived schemes, have resulted in contaminants ending up in the food chain of humans, drawing the attention of the media and the ill will of the public. Because of these concerns, recycling must be accomplished entirely outside the food chain of animals and humans. When this condition is imposed, forest plantations have the advantage over agricultural crops, since the former do not produce any edible product that could carry contaminants. At the same time, plantations are more efficient recyclers, since their «per area» transpiration and their nutrient sequestration are both much larger than those of agricultural crops, grass or pasture.

For the purpose of this article «recyclable effluent» includes effluents resulting from municipal sewers (in cities where the industrial effluents are not discharged in the same system) and animal operations, including milking and slaughter houses, as well as other organic effluents from the food-processing industry.

Among the methods of municipal effluent disposal, recycling this decomposable waste through consumptive use of a forest ecosystem is one of the most ecologically sound. Water is absorbed for the transpirational needs of the tree, while its chemical load is mostly metabolized by the trees. As these systems demonstrate their advantages, especially the lower cost compared to classic activated sludge process, their number will increases. At the same time the safeguards necessary for these open-environment systems have to become stricter.

Such a system is composed by a forest plantation, occupying usually non agricultural-quality land, a storage lagoon and the distribution network (pumps, pipes and sprinklers). As a first approximation, the driving force of the system is the weather, controlling transpiration intensity. The hotter and dryer the weather, the more water the trees take up. Other aspects that have to be considered are soil physical condition, especially internal drainage, and chemical characteristics. Operated on the basis of a uniform clonal plantation, for instance of hybrid poplar clones, the water and nutrient requirements of such a system can be described in sufficient detail to result in a computer model, using weather and a few soil parameters as input data.

Experience with such a recycling system, in many respects conceptually similar to a constructed wetland, in southern Ontario, Canada, allowed for the understanding of operational principles and translation of these into computer rules. The resulting specialized software allows for daily irrigation with effluent of the plantation with an amount of effluent corresponding to the atmospheric demand for evapotranspiration (ETp). It is a flexible instrument that allows for the integration of effluent applications with the rains. This way, the administered effluent cannot surpass the amount that can be transpired in one day. Because the trees absorb water through a deep mesh of fine roots, which reaches 2.5 m in the case of hybrid poplar, and even more when certain willow species are used, the gravitational soil water flow of water cannot develop except when it reaches a very wet condition or if the soil is very permeable. The former condition is avoided simply by not spraying during a rain event, while the later is taken care either by locating the spray area over terrain with at least medium permeability, or by lining it with a plastic film.

The purpose of this paper is to describe in detail a software package that allows the operator of a forested municipal effluent sprayfield to load the field with the maximum amount of effluent allowed by the atmospheric demand for evaporation, determined in real time. All parameters relevant to weather condition and the atmospheric demand for transpiration are archived for further inspection.

Conceptual System

The effluent recycling system uses a clonal poplar plantation as the recycling engine. Such a plantation has a transpiration function, driven by the weather characteristics, and a nutrient sequestration function, steered by the biomass buildup. In the system, the plantation is supplied with water/effluent according to its necessities, determined on the base of five weather parameters monitored continuously every 10 minute: solar radiation, air temperature, air relative humidity, wind speed and rainfall. Monitoring of weather parameters is assigned to a weather station equipped with a micrologger, capable also of data transmission over a phone line. Weather data is retrieved by an on line computer from micrologger and the corresponding evapotranspiration of plantation is calculated for every 10 minute interval. In a daily cycle, these amounts are summed until they reach the amount for a watering, a threshold that is pre-established based on soil texture. After the watering is administered, the summing of evapotranspiration begins again for the next watering.

The pre-established watering threshold has to be determined for each site, being lower when the soil is lighter and greater when the soil is heavier, so as to allow loading of soil with a maximum amount of effluent, yet to prevent soil moisture variations deeper than 1.5 meters, considering a root zone of plantation of 2.5 meters. It is also considered that an increase in soil moisture beyond the lower level of the root zone, 2.5 meters, will not result in a retrieval of water for transpiration, and will be cause infiltration to groundwater.

When operated through the software, this forest eco-technology is able to handle large flows of either primary treated or even raw municipal effluent, without environmental risks. The advantage of operation through this software consists in the continuous loading of the ecosystem with the maximum amount of effluent it can transpire, according to the daily evolution of weather parameters, and allowing for the integration of the rain events in the watering activity.

The effluent recycling plantation

The ecotechnology of recycling municipal waste water through tree plantations is a developing science. Only a few applications exist around the world, and they follow various design approaches and sometimes even have differing objectives. However, the effects of recycling municipal effluent through tree plantations can be summarized to:

In Canada, under a sub-humid climate, irrigation with effluent should be practised only during the dry spells between rains. The strict local environmental regulations prevent groundwater and/or river contamination. In this environment, at the Ontario Forest Research Institute we have defined a new concept of environmentally sound effluent recycling combined with intensive silviculture, which is distinct from the applications already mentioned in the following aspects:

We started a research project based on these requirements in 1983 at Seneca College, in King City, Ontario. The pre-existent effluent disposal system was based on a fixed daily rate of 5.6 mm secondary treated effluent sprayed during growing season over wild grass. The system had primary screening, a large storage lagoon, with an aeration system, and about 22 ha grass area. The effluent was distributed through underground plastic pipes and risers with high pressure sprinklers with a Athrow@ radius of 36 m. The sprinklers were controlled through solenoid valves. To increase the recycling rate, the decision was made in early 1983, to plant the grass area, after proper soil preparation, with poplar clones. In areas prone to waterlogging, willows were planted instead.

The plantation was established with 1-year rooted cuttings, at 4 x 4 m for poplar (clones of Populus x euramericana and Populus deltoides) and at 2 x 2 m for willow, in terrain previously ploughed at 30 cm and treated with herbicides against perennial weeds. During the growing season the area was disked 3 times, against the annual weeds. At the end of the first year, the average height was already 2.5 m. During the second growing season, the plantation became partly functional, and at season=s end the average height was over 4 m. Willow grew slightly slower but developed more foliage. After that, the plantations were maintained only through one disking per year at the beginning of the growing season.

The monitoring hardware

The hardware necessary for this system is composed of the following of-shelf components: a Campbell Scientific micrologger used to scan the weather sensors, a PC and a pump controller able to receive timing commands from PC. The function of PC is of a main driver, communication and timing device, making the program a functional part of the system.

The programs

The system has been operated through a dedicated PC, which was on-line with a weather station. Through a communication loop, executed every 10 minute, it downloaded the weather data and executed the program which has as a primary function the archiving the data.

Description of Program

The program is written in Liberty BASIC version 4, a programming language that allows for the easy integration of data processing with interactive color graphics. The program has been prepared for a screen resolution of 1024 x 768 pixels.

The outputs of this program consist in a) an archive file for weather data, b) an archive file with watering and timing signals for the pump controller, and c) in the possibility of data examination through seven daily graphics. It has to be mentioned that the communication interrupts for data downloading, although frequent - in a day there are 144 intervals of 10 minute - are short enough not to interfere with the operation of the program. This way, a dedicated single-tasking operating system can accomplish these repetitive tasks, without the need for a multi-tasking operating system and of operators with considerable computer knowledge.

For demonstrating the program graphics, two days were selected: July 12, 1993 (day 193), a clear day, and July 26, 1993 (day 207), a cloudy day with rain at dusk. The results are depicted in Fig. 1 to 6 (day 193) and Fig. 7 to 12 (day 207).

The first graphs, Figure 1 and Figure 7, present the daily evolution of net solar radiation (histogram), air temperature, air relative humidity and wind speed. In Fig. 1 a parallel evolution around mid day can be noticed for solar radiation and temperature, and a depression of relative humidity at the same time. The image of Fig. 7 presents more variation of solar radiation, due to cloudiness, and less diurnal variation of air temperature and relative humidity.

The second graphs, Figure 2 and Figure 8, depict the evolution of saturated vapor pressure (a function of air temperature) and of actual vapor pressure. The distance between these lines gives the saturation deficit of the atmosphere, which expresses the capacity of the air to take water vapor. Here the difference between days 193 and 207 is remarkable, the saturation deficit of the atmosphere in day 193 being longer and much more intense than in day 207, allowing for much more evapotranspiration to occur. This makes a strong case for the need to adapt the irrigation rate to the weather conditions that change every day. Finally, at dusk in day 207, we see the apparition of a short rainfall, which nullifies the saturation deficit of the atmosphere.

The third graphs, Figure 3 and Figure 9, illustrate the influence of net solar radiation (histogram) and air temperature (the thermal factors) on the evapotranspiration. It is obvious that ET thermal is considerably greater in day 193 than in day 207. Correspondingly, in day 193 the ET total is greater than in day 207.

The fourth graphs, Figure 4 and Figure 10, present the influence of wind speed (histogram) and saturation deficit of atmosphere (the aerodynamic factors) on the evapotranspiration. It is obvious that ET aerodynamic is considerably greater in day 193 than in day 207, mostly as an effect of greater saturation deficit in that day.

The fifth graphs, Figure 5 and Figure 11, show the daily paths of ET thermal, ET aerodynamic and ET total, as well as of rainfall. This allows the user to see the influence of thermal or aerodynamic factors, which is variable from a day to another, on the evapotranspiration.

The sixth graphs, Figure 6 and Figure 12, present the ET total and the timing of waterings For these graphs, a standard watering rate of 20 m3/ha has been selected. These graphs indicate also quantitatively the amount of effluent administered. A comparison between day 193 and day 207 emphasizes that, when solar radiation and saturation deficit are great, correspondingly, the evapotranspiration is greater and waterings more frequent.

An additional graph (not shown) permits an examination of the most appropriate «per ha» watering rate, allowing to show the frequency of the waterings of different amounts for the evapotranspiration of the same day.

On each graph, on a pink bar under the X axis, there is a representation of the daylight time, which is calculated for every calendar day.

The program is conceived to work in two manners: a) as a continuous system, receiving the weather data every 10 minutes and indicating the timing of waterings as a signal to the pumps, and b) as a system working with archived data of past days, where it allows for comparisons between days.

Why use wastewater for forest irrigation?

When decomposable wastes are recycled through a plantation ecosystem, the principal advantage consists of recycling exclusively outside the food chain. However, several traits of such an operation, detailed below, compound significantly the advantages.

Operational Aspects

Establishment of a recycling system based on a fast growing plantation requires considering the following:

Where does the future of plantations lie?

Transpiration of green plants, which is a function of the climatic demand and of the degree of saturation of the soil reservoir, is greater in semi-arid or arid areas. However, vegetation needed for the recycling process, may not grow naturally in these areas, due to water scarcity. Yet, such areas, often with frequent, desiccating winds, are the most favourable for the application of this concept. In the United States, Arizona, Nevada, New Mexico, Western Texas (especially the border zone) and Southern California are areas characterized by the greatest values of ETp, high insolation and a long growing season. As a result, effluent recycling can be most effectively applied. Establishing forestry crops that consume large water quantities does not put any special problem in such areas, as long as enough municipal effluent is available for irrigation. The main advantage of this system based on the natural processes of transpiration and nutrition remains its ability to recycle water only to the atmosphere, instead of disposing it to the groundwater.

Effluent recycling is an entirely natural technology, based on sound environmental principles. It is also cheap. Moreover, through both service to the community and timber production, it can be made profitable. Despite these great prospects, some questions remain unanswered. We need to learn more about how effluent recycling performs under field conditions, especially in arid areas and with more species. We must also recognize that the tremendous population increase in some poor areas of the globe has deepened the poverty and raised the risks of epidemics to new dimensions, adding a sense of urgency to wastewater recycling research. These critical problems demand the attention of scientific community and national and international funding agencies. Where conditions permit, this forest ecotechnology must be considered as a component of a large, integrated approach, able to stop the deterioration of numerous living systems and as a means for reclamation.

Figures

  1. 10 minute variation of Temperature, Net Radiation, Relative Humidity and Wind
  2. 10 minute variation of Temperature, Saturated Vapor Pressure, Actual Vapor Pressure and Rainfall
  3. 10 minute variation of Temperature, Net Radiation, ET thermal and Total Evapo-Transpiration
  4. 10 minute variation of Wind, Saturation Deficit, ET aerodynamic and Total Evapo-Transpiration
  5. 10 minute variation of ET thermal, ET aerodynamic, Total Evapo-Transpiration and Rainfall
  6. 10 minute Total Evapo-Transpiration, Rainfall and Watering Rates
  7. 10 minute variation of Temperature, Net Radiation, Relative Humidity and Wind
  8. 10 minute variation of Temperature, Saturated Vapor Pressure, Actual Vapor Pressure and Rainfall
  9. 10 minute variation of Temperature, Net Radiation, ET thermal and Total Evapo-Transpiration
  10. 10 minute variation of Wind, Saturation Deficit, ET aerodynamic and Total Evapo-Transpiration
  11. 10 minute variation of ET thermal, ET aerodynamic, Total Evapo-Transpiration and Rainfall
  12. 10 minute Total Evapo-Transpiration, Rainfall and Watering Rates

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