The first article in this series provided a broad overview of the indoor cultivation and production of cannabis, describing the typical components and key infrastructure systems required for a successful operation. This article expands upon the importance of water to these unique and challenging facilities.

Cannabis and other indoor farming operations are highly dependent on an adequate and quality water supply. These facilities use water for many purposes; the most important and largest usage is plant irrigation.

“Cannabis H₂O: Water Use & Sustainability in Cultivation,” a 2021 report published by the Research Innovation Institute (RII), New Frontier Data and the Berkley Cannabis Research Center, found that, on average, indoor cannabis facilities use 209 gallons/square foot/year for irrigation (https://bit.ly/3vHs93O). Other noncultivation uses include: 

• Evaporative cooling applications, such as pad and fan systems in naturally ventilated greenhouses and cooling towers for water-cooled indoor growing operations. Evaporative cooling systems require a constant supply of makeup water to offset the moisture that has evaporated and been lost to the atmosphere. 

• Humidification of indoor grow rooms and post-harvest spaces such as drying and curing rooms.

• Cooking and dishwashing equipment in commercial kitchens for the preparation and production of edible cannabis products.

• General domestic water uses including employee welfare spaces, showers and locker rooms, onsite laundry, cleaning, emergency showers/eyewash stations, etc. 

Irrigation

The most important application of water for indoor cannabis facilities is plant irrigation. The first challenge is determining the amount of water required for this purpose. Typically, the plumbing designer will collaborate closely with the facility’s cultivation team or head grower to develop an understanding of the total irrigation program. 

This includes the quality of water required, how water will be delivered to the plants, watering frequency, and the amount to be delivered during a certain period. Watering rates are often described using a metric of gallons per plant per day, or gallons per square feet of plant canopy per day. 

Equipment can be sized and selected once the total daily irrigation quantity is understood. The starting point is usually filtration and purification. Most cannabis operations will use a reverse osmosis (RO) system to remove contaminants from the water supply. According to the Cannabis H₂O report, RO is especially helpful in cannabis as it is one of the only ways to remove sodium and heavy metals from the plants. 

With the stringent testing requirements for cannabis products, growers err on the side of caution by using this treatment method. The RO system should be sized to provide the minimum daily output for the facility, and storage tanks should be sized for at least one day’s worth of irrigation to provide a buffer for any hiccups in operation. 

Incoming water from the utility should be pre-heated to room temperature for two purposes: to improve the efficiency of RO membranes, and ensure that cold water is not fed directly to plants (think of taking a cold shower). RO water is pumped from tanks to nutrient injection and mixing equipment, with the resulting fortified water known as fertigation. 

Fertigation water is stored in batch or day tanks or pumped directly to grow rooms. Most operations will use separate recipes or batches for each type of grow room or stage of plant growth. Each grow room will have one or multiple zones, with each zone controlled by a solenoid valve. 

Inside the grow rooms, fertigation piping will be routed to each zone, which generally consists of multiple plants on a rack or bench, with one or more drip emitters serving each plant (see Figure 1). In general, watering events are set up as reoccurring schedules or can be initiated by a signal from soil moisture sensors. 

Irrigation and fertigation systems can range from relatively simple to complex and fully automated. Small operations may water plants by hand. Other facilities may use a small residential-type irrigation controller and an inline nutrient injection device (see Figure 2). 

More sophisticated operations may use pre-packaged, skid-mounted equipment with integral pumps and controllers to generate RO water, produce fertigation water and deliver water to grow rooms (see Figure 3). Figure 4 shows a flow diagram developed for a large cannabis facility. 

Equipment and tanks for irrigation/fertigation can be large and require a fair amount of floor space and ceiling height. Floor space is also needed to store and stage bulk nutrient totes or drums. Therefore, it is important to work with the owner’s team to determine the program for the irrigation and fertigation systems as early in the process as possible. 

Also, make sure there are large enough pathways and doorways to get large tanks from the exterior of the building to the irrigation/fertigation room — a lesson that, unfortunately, may have been learned the hard way by some! 

Piping material for RO and fertigation water should be selected to handle the characteristics of purified water and withstand ultraviolet light produced from grow lighting inside the grow rooms. Irrigation and fertigation equipment and controls may need to be on the facility’s emergency power system to avoid service disruption during a power outage. 

Drainage and Wastewater

The collection of excess irrigation/fertigation water (runoff), or leachate, is often overlooked early in the design process but can significantly impact grow room and drainage system layouts. Many growers are using moveable benches or racks to improve floor area efficiency. Some benches include integral trays that collect excess irrigation water and slope to a drain port. Other benches slope continuously from end to end. Benches and racks may have single or multiple tiers/levels. 

No matter the setup, plumbing designers must coordinate leachate drain piping with the bench/rack equipment selected by the owner to maintain working clearances and prevent trip hazards. Collected leachate is typically directed to a trench drain or floor drain in the grow room, or to a sump or condensate pump if reclamation is being considered. 

The amount of condensate from heating, ventilation, air conditioning and dehumidification (HVACD) equipment is substantial in indoor grow facilities. Irrigation water delivered to plants ends up as excess as described previously or will be “exhaled” by the plants through the evapotranspiration process. 

Around 80% to 95% of irrigation water will be transpired by plants, which means moisture will be handled by the HVACD systems serving the grow rooms and discharged in the form of condensate from cooling coils and dehumidifiers. A fair amount of condensate also is produced from HVACD systems serving drying and curing rooms. Most facilities operate year-round, so any condensate drains on exterior HVACD equipment will need to be protected from freezing conditions. 

If a facility is served by a municipal sewer system, the design team should verify with the utility that the wastewater from the facility can be accepted into the system without pretreatment. If it is served by an onsite septic system, designers should evaluate the potential impact of the amount of backwash from the RO equipment on the septic system capacity, as well as the impact of the constituents of waste fertigation water on the septic system performance. 

Water-Efficiency Opportunities

Indoor cannabis facilities have several opportunities to reduce water usage or reclaim water. First, growers can use drip irrigation systems that have been shown to reduce water consumption by 30% to 70% and improve water productivity by 20% to 90%. Adding sensor-based controls to a drip irrigation system can further reduce water usage. These statistics are explained in the Cannabis H₂O report. 

The vast majority of condensate from HVACD equipment can be captured and reclaimed, greatly reducing the amount of raw water required for irrigation or other needs. To be reused for irrigation, collected condensate will need to be stored in a tank(s) and should be retreated through the RO system to remove any contaminants. 

Leachate is another waste stream that can be considered for recycling but typically does not make financial sense in smaller operations. Leachate is generally collected in floor or trench drains in each grow room and will need to be pumped back to the water treatment room and stored in a separate tank(s). From there, it will need to be routed through a separate treatment skid before being reintroduced as irrigation water. 

If operations use cleaning solutions in grow rooms between crop cycles, the leachate treatment equipment may need to be enhanced to process the additional chemicals in the waste stream. 

Additional system controls will be required when using a combination of raw water, condensate or reclaimed leachate water for irrigation purposes. 

Facility Water Service Sizing

Irrigation needs will generally dictate water service sizing for an indoor cannabis cultivation facility. However, makeup water for evaporative cooling systems could also impact the total amount of water required. 

My firm recently was involved with designing a very large indoor cannabis operation in a rural area and found out midway through the design process that the local water utility did not have the capacity to serve the proposed cooling towers and irrigation requirements. Therefore, we had to change the cooling approach for the facility. 

The lesson here is that the design team should engage the local water utility early to understand water quality issues and ensure capacity is available for the proposed project. 

Part three in this series will explore HVACD systems and air distribution for indoor cannabis cultivation facilities.

Luke Streit, PE, is a project executive and mechanical/process engineer for IMEG Corp., where he has led the firm’s growing portfolio of controlled environment agriculture facility design. He also has an agricultural engineering degree from Iowa State University, was named an ENR Midwest 2020 Top Young Professional, and belongs to several industry organizations, including ASABE, ASHRAE, NFPA and RII.