Greenhouses have a long industrial history. All over the world, humans have used greenhouses to boost the production capacity of fruits, vegatables, flowers and many other consumer products. The challenges associated with greenhouses have been and still are the control of microbial growth and the use of pesticides and biocides to avoid product spoilage.

In response the environmental challenges caused by pesticides emissions, new laws and guidelines are being introduced and enforced upon the greenhouse industry. In short, new regulations comprise:

• Reuse of water in hydroponic greenhouses
• Elimination of pesticides in the greenhouse wastewater
• COD (Chemical Oxygen Demand) treatment in the wastewater

Challenges in hydroponic greenhouses

Hydroponic production is the major technology used by the industry today. Crops are cultivated soil-free. Nutrient is added directly to the water supply. Nutrient addition to the water supply also generates a favorable environment for mold and bacteria in pipes and other surface areas. Microbial growth must be controlled to mitigate pathogens, hence a number of disinfection methods are required to keep product spoilage, deceases and bacteria at bay. Common methods are organic pesticides, heat, hydrogen peroxide, hypochlorite, UV, and ozone.

Many of these methods and technologies may cause secondary problems and environmental compliance issues related to the greenhouse effluent water, environmental health and safety (EHS) concerns, intense consumables costs, energy demand and extensive maintenance. An overview of various disinfection methods that can be used for greenhouse recirculated water, wastewater and irrigation is presented below.

Ozone treatment in greenhouse operations

Water recirculation and reuse in greenhouse has several operational advantages and is associated with significant cost savings. As irrigation water is used to grow crops, a fraction of the volume is not imbibed by the crops. Depending on the type of greenhouse and the type of crops being cultivated, the fraction of excess irrigation water is around 20-40%. New EU regulation stipulates that this volume must be reused in subsequent irrigation cycles. This presents a number of challenges for the grower since circulated water may be subject to microbial contamination. It is also vital to be able to retain the nutrient content. Ozone is an attractive method thanks to its highly potent disinfection capacity and its residue-free characteristics. It is also an excellent approach to treating wastewater containing elevated levels of organic COD and pesticide residues.

Greenhouse water reuse with ozone

As the non-absorbed water volumes are drained from the crops, a recirculation loop pumps the used water back to a collection tank. The annual water demand is typically 3,000-12,000 m³/ha of greenhouse production area without water circulation, depending on the type of crops. The collected water must be thoroughly disinfected in order to avoid disease from microbes or pests. Ozone opens up the possibility for maximizing water reuse rate and minimizing the nutrient consumption. A typical reuse cycle is described in the image below.

The key advantages associated with ozone treatment and sanitation comprise:

• Fast disinfection. Sanitation can be performed with in 5-10 minutes in batch mode.
• Low energy input. Compared to heat sanitation, required power input is normally only 1-3 kW.
• Low footprint. Ozonetech's RENA Vivo water systems are designed to fit onto a standard EU-pallet.
• Treatment of unwanted organic residues. Ozone effectively reduced the organic COD and BOD loads of the water.
• Nutrients such as ammonium, phosphate, sulphur and potassium remain unaffected. 

There are tangible savings to using ozone compared to heat or chemical use for disinfecting reuse greenhouse water. As an example, if 10,000 m³ of water is reused annually, up to USD 20,000-25,000 (EUR 18,000-23,000) can be saved on energy or fuel used to heat water up to 95 deg C. For facilities using traditional biocides, up to USD 5,000-10,000, excluding handling and administration costs, can be realized.

More information about how ozone is used to sanitize water, please read more here.

Ozone irrigation disinfection

Commonly, additional chemicals such as hydrogen peroxide or hypochlorite are dosed into the irrigation water flow to prevent microbial growth in the system piping. Pesticides are sometimes used to prevent pests. Ozone replaces sanitation chemicals at concentration below 1 ppm. A properly designed ozone system can easily be incorporated into the irrigation flow or at the normally positioned chemical dosing point. Ozonated water will not damage or inhibit growth of the crops. A description is given in the image below.

Ozone is broad range biocide and pestice, completely inorganic and gentle to process equipment and crops. Hence it can be used to replace pesticide use if required.

Ozone wastewater treatment

In spite of the major advantages to water reuse, the irrigation system is normally emptied and cleaned seasonally. At this time, pesticides (if applicable) and potentially elevated levels of BOD and COD need to be treated along with new regulations, or to avoid heavy wastewater discharge to local wastewater treatment plants. The process of BOD and COD treatment using ozone is described here. If pesticides are used, they can be eliminated from the wastewater through chemical oxidation through first and second order reactions. Pesticides are highly toxic compunds that may disrupt the biologial process and are subject to bioaccumulation if released into the environment due to their stable chemical structures. 

Ozonetech has extensive experience in treating pesticides from actual greenhouse wastewater. An overview of a few representative pesticedes are illustrated below:

Pesticide	Chemical structure
Cyprodinil This compund is non-chlorinated but chemically stable. Ozone degradation occurs as the aromatic structures are oxidized through the potent oxidation potential of ozone, even at very low concentrations. Free electrons around the nitrogen atoms also makes this pesticide prone to ozone degradation. Degradation occurs by a second order reaction. Cyprodinil is an effective fungicide.
Imidacloprid This pesticide is partially protected by the chlorinated structure, which may give slightly slower degradation rates at zero order. This compund is used as an insecticide.
Penconazole The chemical structure has two chloronated parts, improving its resistance to chemical break-down. The chlorination structure forms a physical protection from oxidative compounds like hypochlorite and ozone. However, with ozone treatment, this fungicide is degraded completely at zero order reaction. Acts as a fungicide in greenhouse operations.
Pirimicarb Pirimicarb as an insecticide and has a complex chemical structure with multiple methyl groups bound to aromatic structures. Still it degrades quickly using ozone by first order kinetics.

Normally, a balanced mix of pesticides are used to cover a broad range of pests, all of which can be broken down by ozone in the wastewater stream.

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