From a grower’s perspective, water quality and quantity (i.e. good quality, consistent supply) are key factors in the operation’s ability to produce high-quality plants and products. Cost-effective water treatment systems are especially important to growers with poor-quality water sources, farms that need to meet regulatory nutrient discharge limits, or farms that are trying to maximize their water-use efficiency by recycling operational water. Treatment systems should also be customizable to meet their specific needs, while requiring minimal maintenance.
With these challenges in mind and with financial support from both Agriculture and Agri-Food Canada (AAFC) and the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA), we set out to design and test innovative technologies to help farmers manage their operational water. The project included designing and testing systems that combined woodchip denitrification bioreactors, selective mineral media and a constructed wetland design to create a true hybrid treatment system (HTS). The ultimate goal was to develop custom greenhouse and nursery water treatment systems that not only protected the environment but also enabled growers to recover and reuse operational water, improving water and nutrient-use efficiency without risking crop health.
There were three parts to this project:
- The construction of two pilot-scale treatment systems to test various media and designs
- The installation of two permanent HTS to treat floriculture greenhouse and container nursery operational waters, using information gained from pilot studies
- The development of a detailed guide to help growers select and design site-specific water treatment technologies
The pilot systems (Fig. 1) were used in two types of short-term studies. The first was to test different media types individually over time (known as “batch runs”). The second placed several media types in a row and ran water through the cells one after the other (Fig. 2), checking the water quality after each cell (known as “series runs”). Media types included woodchips, pea gravel, filter sand, slag and wollastonite – all designed to remove selected contaminants such as nitrate-nitrogen, phosphorus and pathogens. Our research tested water at low, medium and high nutrient concentrations and at various flow rates.
Here were some of our key results:
- Removed nearly all of the nitrate-nitrogen under anaerobic (oxygen-free) conditions
- Removed 50 to 60 per cent of the phosphorus when concentrations in the untreated influent water were relatively high (greater than 10 parts per million)
- Effectively removed fungal plant pathogens (such as those measured by standard DNA Multi-Scan™) with sufficient residence time under anaerobic (oxygen-free) conditions
- Slightly increased biological oxygen demand (BOD), but this was reversed by the mineral media treatment used afterwards
- Alkalinity increased by up to 125 ppm; the impact of higher pH to any nutrient or chemical inputs should be considered before reusing treated water in the greenhouse or discharging it
- Were both effective in removing phosphorus
- Downsides of these types of media could be a) the limit of the media for phosphorus removal is still unknown and b) an increase in water pH when using slag
Effects on residence time:
Lower flow rates mean longer residence times, so the water to be treated has more time to interact with the filtering media.
- If the residence time is too short, then insufficient nutrient or pathogen removal occurs
- If the influent contains higher concentrations of contaminants, residence time needs to be increased to achieve effective treatment
To address the need to remove phosphorus, we tested the woodchips in series with a range of mineral media including pea gravel, filter sand, wollastonite and slag. The slag/gravel mixture was most effective at phosphorus removal but increased the pH of the treated water. Wollastonite effectively removed phosphorus, as did filter sand, but their maximum binding capacity is still unknown (the media in the pilot systems still has binding potential). Pea gravel also removed some phosphorus, but we don’t expect that to be effective for long-term installations.
- FIGURE 1. Black media cells FIGURE 1. Black media cells
- FIGURE 2. Schematic of portable treatment system cells FIGURE 2. Schematic of portable treatment system cells
- FIGURE 3. Installation at container nursery FIGURE 3. Installation at container nursery
- Table 1. Summary of treatment results Table 1. Summary of treatment results
- FIGURE 4. Installation at floriculture facility FIGURE 4. Installation at floriculture facility
Putting our pilot results to the test in permanent HTS installations
Two permanent systems were installed during this project: one at a container nursery (Fig. 3) and another at a floriculture greenhouse producing potted plants (Figs. 4). There is sufficient data at this point to confirm that the woodchip cells were effective in removing nitrate-nitrogen and pathogens. Similar to the pilot studies, the woodchip cell only removed significant amounts of phosphorus if the influent (incoming) water contained levels higher than 10 ppm. For low influent concentrations, the woodchips had little to no effect on phosphorus levels (Table 1).
Both cooperating facilities are able to use the surface of the HTS as a growing area for potted nursery stock and garden mums, but it is not advised to drive on the HTS with machinery. These systems require some maintenance to ensure that pumps are working, and alarms have been installed and connected to the greenhouse Argus automation system. Otherwise, the systems are anticipated to last approximately eight to 10 years without any significant maintenance. The entire system does take a few months to start running at full capacity, but should continue to perform consistently when temperatures are above 10 degrees C. This is critical for the woodchip cell as it contains temperature-sensitive bacteria essential to the denitrification process. Additional woodchips may be required to ‘top up’ the system after several years. The cost of these treatment systems ranged from $60,000 to $100,000 to treat a range of 25,000-200,000 L/day, plus additional costs for electrical and isolation/storage of the water before and after treatment.
The HTS represents a customizable tool for water treatment, particularly in situations where there is a need to recirculate or discharge very clean water. While these systems do require a significant footprint outdoors, but not necessarily a loss of production space, they can be tailored to match the volumes and fluctuations of a particular operation. In fact, they can handle stormwater variations in addition to greenhouse operational water inputs. Self-assessment of the farm is highly recommended prior to choosing a water management solution.
The Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) has prepared best management/self-assessment workbooks for greenhouse floriculture, vegetable and container nurseries (www.ontario.ca/publications). For assistance in determining if an HTS or another technology is appropriate for treating your water, a guide has been developed for growers. Find this, along with factsheets and more information on HTS and variations on the technologies used in this study on the Flowers Canada website (www.flowerscanadagrowers.com/environment-water-specialist-resource-page).
Key terms you should know
Residence time = Also known as hydraulic retention time, this is a measure of how long the water is held in the treatment process. This changes with the flowrate of the incoming water.
Biological oxygen demand (BOD) = Oxygen is needed for microbes to break down organic matter. This “demand” is used as a way to measure levels of organic matter in the water.
Influent = Incoming flow of water into the treatment process.
Woodchip denitrification bioreactor = A container of woodchips under saturated (anaerobic) conditions in which a population of bacteria develops that removes the oxygen from nitrate (NO3-), converting it to N2 gas. This is called denitrification.
Wollastonite = Calcium inosilicate (CaSiO3), a mined mineral that’s been shown to bind phosphorus.
Slag = A product of the steel-making process shown to bind phosphorus.
The authors would like to acknowledge funding for this project by Agriculture and Agri-Food Canada through the Canadian Agricultural Adaptation Program (2014-2019), as well as support from OMAFRA. The views expressed in this article are the views of the research team and do not necessarily reflect those of their funding partners.