A deep dive into the methods which cannabis cultivators have successfully deployed to dispose of fertigation runoff and other cultivation facility waste streams  

Introduction 

At the time of writing, 37 states have legalized cannabis for medical use and another 18 states have legalized recreational cannabis. The widespread legalization of cannabis for use in medical and recreational applications in many states has created a booming new industry. Local businesses growing and selling cannabis are springing up to meet market demand. The cannabis cultivation process begins with growing the flower in either an outdoor or indoor growing facility. 

Outdoor cannabis cultivation is very similar to traditional farming in fields with a single crop produced each growing season. Outdoor growers are at the mercy of mother nature just as farmers are.   

While outdoor growing may be viable in some environments, many if not most commercial scale cannabis cultivation operators are utilizing indoor cultivation to maximize crop yields and control of the process. Many of the indoor cultivation concepts have been developed based on the practices developed by traditional indoor produce cultivators who have been optimizing these methods for many years. In addition to the benefit of multiple harvests per year, better yields, and consistent quality of product; by growing indoors, cultivators can control: 

• Lighting 
• Temperature 
• Carbon dioxide levels 
• Nutrient levels 
• Insect predation 
• Viral and bacterial diseases 
• Access and security of the crop 

However, nothing is ever free in this world and indoor cannabis production is no exception. Drawbacks include: 

• Much higher production costs 
• Large energy / electrical consumption for artificial lighting 
• Need for a building with heating & cooling ventilation systems  
• Replacement soil costs 
• Risk of disease, resulting in large crop losses  
• Waste disposal costs for solid waste and wastewater 

In this paper, we’ll explore one of the primary challenges of indoor cannabis cultivation, wastewater treatment and disposal. 

Indoor Cannabis Cultivation methods 

There are several indoor cannabis cultivation techniques. All of the techniques have advantages and disadvantages that growers need to be aware of.    

• Ordinary soil 
• Coco Coir Fiber 
• Hydroponic 
• Aeroponic 
• Aquaponic 

Wastewater Sources 

Typically, wastewater produced by an indoor cannabis cultivation facility is considered an industrial waste. The classification of industrial wastewater places a greater burden on the grower to ensure all regulatory requirements are being met. 

Cultivation wastewater can be generated from several sources: 

• Nutrient runoff from plants 
  • Contains dissolved nutrients and suspended soil materials   

• Reject streams, such as RO reject, from raw water treatment processes which make water suitable for fertigation of the cannabis plants 
  • Contains salts and impurities found in raw water 

• Facility cleaning and sterilization 
  • Contains cleaning compounds such as bleach, peroxide, soaps and other industrial cleaning chemicals 

• HVAC condensate 
  • Contains low levels of impurities extracted from the air in the facility  

• Storm water 
  • Contains dirt, debris, oils and trash that rainfall washes off parking lots and buildings 

The combination and ratios of these various sources of wastewater will be different for all growers and locations. Since all growers will have a different wastewater, a single standard treatment method becomes more difficult to achieve.  

Wastewater Treatment Options 

Executive Summary

OPTION 1: Publicly Owned Treatment Works (POTW) 

Discharging the wastewater to a local POTW (aka “the drain”) is generally the most cost-effective option for treatment, if permitted and accessible. Cultivation wastewater is combined with the sanitary sewer wastewater and sent to the POTW via underground piping. The POTW facility will typically remove solids and send the wastewater through a biological process to breakdown organic materials and nitrogen compounds before discharging the treated wastewater to the environment. Billing is typically based on volume treated with surcharges for particular pollutants such as TSS and BOD.  

Limitations to this option are: 

• Location of the POTW to the indoor grow facility to allow pipeline connection 
• License restriction of the POTW to accept cannabis cultivation wastewater into the POTW facility. Typical license restrictions are wastewater classification (industrial/sanitary), TSS, BOD/COD/color.  
• Discharge permit restrictions on the POTW for discharge of certain pollutants that will pass through the POTW facility into the receiving body of water 

In many cases, even if certain waste streams such as HVAC condensate may be permitted by a municipality to be discharged to the sewer, other waste streams such as fertigation runoff which has higher contaminant loading may not be permitted for discharge. 

OPTION 2: Direct discharge from the cultivation facility to the environment 

Direct discharge refers to the discharge of wastewater from the plant to the local environment – ground application, discharge to a local water body, etc. Direct discharge will require a National Pollutant Discharge Elimination System permit (NPDES). NPDES permits follow guidelines established at the federal level with additional state and local requirements. NPDES permits are issued by states under authority of the federal EPA. 

Typically, permits will require treatment and monitoring programs to be implemented in order to meet the requirements for safe direct discharge by the facility. Meeting these requirements tends to be very expensive and exposes the facility to civil and criminal liability if compliance is not met. Depending on the body of water being discharged into, some pollutants (phosphorus and ammonia for example) may create significant water quality issues such as algae blooms for the receiving water body if they are limiting nutrients.  

 

OPTION 3: Hauling of wastewater 

Hauling of wastewater is the process of accumulating the wastewater in tanks on site and then trucking the wastewater to an approved facility for treatment and disposal. This is a very common and convenient method for dealing with the facility’s cultivation wastewater, though it is almost always prohibitively expensive. The wastewater hauler will classify the wastewater and establish a hauling rate that typically includes trucking costs and treatment of the wastewater. Convenience does come at a price, as costs for hauling wastewater range from $0.40 to $2.50 per gallon. Hauling is 200 to 300 times more costly than disposal to the local POTW.  In most cases, cultivation facilities will substantially reduce the volume of their wastewater via conservation, evaporation, or reverse osmosis (RO) prior to hauling to minimize their overall costs.  

Hauling, NO Volume Reduction

For a growing facility that generates 1,000 gallon per day of wastewater and does not utilize any volume reduction techniques, hauling would cost in the range of $400 to $2500 per day. Over the course of a year, hauling of wastewater would cost a grower between $146,000 to $912,500 per year in disposal costs.

 

OPTION 4: Volume Reduction 

Volume reduction is a cost saving technique that is used to reduce the volume of wastewater needing to be hauled away from a facility for disposal. Volume reduction can be achieved in many different ways.  

• Changing production & maintenance processes to increase efficiency of water usage in facility 
• Recycling the water via filtration technologies such as reverse osmosis 
• Separation of the water from the waste portion via evaporation or reverse osmosis   

Increasing efficiency of water usage 

Typically, increasing the efficiency of water usage means employing strategies that reduce consumption of water. Similar to how a household might use high efficiency shower heads, a hydroponic facility might reduce the amount of excess water that is provided to the plants or change the method of cleaning a grow room after a cycle. Unfortunately, there is only so much that can be gained from efficiency without sacrificing the production rates of the plants and most sophisticated commercial grow operations are already using highly efficient irrigation/fertigation techniques.   

Recycling 

Direct re-use of wastewater without treatment 

Recycling involves re-using wastewater for another purpose in the hydroponic facility. One method would be to re-apply the fertigation runoff back to the plants. While this might allow the plants another opportunity to utilize the nutrients and the water contained in the wastewater, this is a risky and uncommon approach.  The recycling of wastewater presents an opportunity for diseases to be introduced to healthy plants risking an entire crop. For high value crops like cannabis this is a risky proposition as the value of a single harvest could easily dwarf the savings generated by recycling.  

Re-use of wastewater after treatment  

Because re-use of wastewater without treatment is seldom possible, utilizing a wastewater treatment/recycling system to separate the water from the contaminants is typically required.  Common treatment/recycling technologies include reverse osmosis (RO) and evaporation/distillation. Water generated by these processes is typically very clean and easily sterilized to ensure no disease contamination occurs.  

Typical wastewater generated at a hydroponic facility is more than 99.8% water (typical TDS is 1000-1500 mg/L). This means that if a facility is hauling this wastewater off site, they are hauling mostly water. Hauling water is extremely costly and inefficient. Most savvy industrial and agricultural facilities that generate wastewater separate the water from the waste prior to hauling. This is typically done via membrane processes such as Ultra Filtration (UF) & Reverse Osmosis (RO) and/or a thermal evaporation process. Thermal evaporation processes have the capability to reduce the wastewater volume to dry solid waste resulting in a Zero Liquid Discharge for the facility. Facility wastewater volume, hauling costs and associated capital and operating costs will typically dictate technologies that will be used in the final design. Many times, a combination of these technologies will be utilized to minimize CAPEX and OPEX.       

WASTEWATER VOLUME REDUCTION / RECYCLING TECHNOLOGY OPTIONS 

Finally, we’ll explore the various wastewater volume reduction and recycling technologies that are most appropriate and cost effective for cannabis cultivators. 

Membrane Technologies 

Membrane processes are typically used in higher volume applications (greater than 5,000 - 10,000 gallons/day) to provide a reduction in wastewater volume. Membrane process will typically consist of an Ultrafiltration (UF) system followed by a Reverse Osmosis (RO) system.  

The purpose of the UF system is to remove suspended solids from the wastewater prior to introduction of the wastewater into the RO, as the suspended solids will cause operational issues with the RO. No significant reduction in wastewater volume occurs in the UF system. 

RO systems utilize the principle of osmotic pressure, where water with high TDS wants to flow to water with a low TDS. This is how plants accept water and nutrients from the soil. In RO systems, the osmosis process is reversed by applying high pressure to the high TDS side of the membrane, this forces the water to flow through the membrane as clean water and leaves the dissolved solids in the rejected wastewater. Typical wastewater RO systems can recover an average of 65% of the incoming wastewater flow, resulting in an RO reject flow of 35% with a 3X increase in TDS. RO systems are limited by the operating system pressures and scaling tendency of the wastewater. An RO system processing 1,000 gallons per day of raw wastewater will produce 350 gallons per day of RO Reject for disposal or further volume reduction via evaporation. Typical OPEX costs for RO systems are $0.02 to $0.03 per gallon of permeate produced.  

Membrane Technologies Only

For a growing facility that generates 1,000 gallon per day of wastewater, membrane treatment would reduce hauling costs from a range of $400 to $2500 per day without RO to $140 to $875/day with RO at a typical OPEX of $13/day. Over the course of a year, RO treatment can generate savings between $90,155 to $588,380 per year.

 

Thermal Evaporation 

Thermal evaporation is a process where the wastewater is brought to a boiling temperature producing water vapor(steam). As the steam leaves the wastewater, the contaminants in the wastewater are left behind in the evaporator in ever increasing concentrations until an end point concentration is reached. Wastewater evaporators operate as a semi-batch process, the evaporator is filled with wastewater and brought to a boil, water is removed by boiling, reducing the liquid level in the evaporator. More wastewater is added to the evaporator to maintain level. The fill, evaporate, fill process is repeated until an end point concentration is reached. Once the end point is reached, the concentrated wastewater is then discharged to a concentrate holding tank for disposal or further volume reduction in a dryer. The thermal evaporator is then able to start processing another batch of wastewater. 

Thermal evaporators can utilize many different energy sources to supply the heat required to evaporate the water. Natural gas is most commonly used, however propane, electric and steam can also be utilized.   

Thermal evaporator systems have the option of either exhausting the vapor to atmosphere or condensing water vapor into a clean condensate for re-use. Typically, water vapor exhausted to atmosphere is exempt from permitting as any volatiles present in the wastewater that could be potentially emitted are diminutive and below any permitting thresholds. Water vapor exhausted to atmosphere eliminates the need to either utilize or dispose of the water removed from the wastewater. For growers located in areas of water scarcity, the ability to produce a clean water source for re-use can be very beneficial. 

An advantage of the thermal evaporation process is the ability to process wastewater from low to high incoming TDS and TSS levels.  Reverse Osmosis is limited to low incoming concentrations and creates a much more dilute end product. Thermal evaporation results in a concentration factor of 167X to 250X from a typical original starting TDS of ~1000-1500 mg/L. A thermal evaporation system processing 1,000 gallons per day, would produce a concentrated wastewater volume of 4 to 6 gallons per day for disposal or further volume reduction. Typical OPEX for a thermal evaporator is $0.06/gallon evaporated.  

Evaporation ONLY

For a growing facility that generates 1,000 gallon per day of wastewater, evaporation would reduce hauling costs from a range of $400 to $2500 per day without evaporation to $8 to $50/day with evaporation at a typical OPEX of $59/day. Over the course of a year, evaporation can generate savings between $121,618 to $872,788 per year.

 

One of the big advantages of thermal evaporators is that can be used as the entire primary volume reduction piece of equipment or be used to process RO reject as part of a two-step volume reduction process. Generally, this determination is based on the total volume of wastewater needing to be processed. Small volumes do not warrant the extra capital and complexity required for two systems. Large volumes make a better case for upstream volume reduction with Reverse Osmosis. 

Recycle via Treatment

For a growing facility that generates 1,000 gallon per day of wastewater, recycle via treatment (membrane technologies combined with evaporation) would reduce hauling costs from a range of $400 to $2500 per day without volume reduction to $8 to $50/day with evaporation at a typical OPEX of $33/day. Over the course of a year, the recycle via treatment approach can generate savings between $131,108 to $882,278 per year.

 

Zero Liquid Discharge (ZLD) 

Zero Liquid Discharge is the process of removing the water portion of the wastewater until there is no free liquid present. This allows the solids that remain to be disposed of as solid waste or re-purposed. Disposing of solid waste is typically a much lower cost than liquid disposal. Solid waste typically needs to pass the paint filter test to be disposed in a landfill. 

The equipment used for ZLD are called Slurry Dryers. Slurry Dryers will take the liquid wastewater to a dry landfillable condition. To get the wastewater to a dry condition requires the wastewater to be evaporated and concentrated past the solubility limits of the dissolved solids in the water. This process causes the dissolved solids to precipitate out of solution forming a slurry. The slurry is then further concentrated to remove the free water making it landfillable. 

The presence of significant amounts of solids and very concentrated brine need to be addressed in the design and materials of construction. Materials of construction for slurry dryers typically start at the super stainless level and can increase to exotic alloys high in nickel and molybdenum to achieve the required corrosion resistance. The presence of solids also requires a mixer to be used as the wastewater goes from a liquid to a slurry to a flowable dry solid. Oversized valves are used for discharging the dry solids into hoppers. Due to these design requirements, CAPEX per gallon of evaporation is higher with a Slurry Dryer than either RO or thermal evaporation systems.  For this reason, the Slurry Dryer typically is utilized as a final step in the ZLD process so that it can be sized for the low volume of residual liquid discharged from a thermal evaporator system to minimize total CAPEX required.  

Recycle via Treatment with ZLD

For a growing facility that generates 1,000 gallon per day of wastewater, recycle via treatment with ZLD (membrane technologies combined with evaporation & drying) would reduce hauling costs from a range of $400 to $2500 per day without volume reduction to $8 to $50/day with evaporation at a typical OPEX of $26/day. Over the course of a year, the recycle via treatment approach can generate savings between $131,108 to $882,278 per year.

 

Conclusion 

Wastewater disposal from indoor cultivation of cannabis needs to be addressed in a manner that is both cost effective and environmentally sound.  

Typically, if the cultivation facility has the option to discharge wastewater to the POTW, that is the most economical and simple choice. 

When connecting to the POTW is not possible, reducing the volume of wastewater via Reverse Osmosis (RO) and/or evaporation prior to hauling wastewater to an approved treatment facility becomes the next best solution. Once the decision to haul wastewater is made, minimization of wastewater disposal costs becomes the focus of the cultivator to minimize expensive hauling costs.  

High volume facilities will typically install Reverse osmosis systems followed by evaporation systems to process the reverse osmosis reject to minimize overall CAPEX and OPEX.  

Smaller facilities will utilize evaporation only to minimize CAPEX and simplify process into a single step. As smaller facilities grow and their wastewater generation increases, a reverse osmosis system can be easily added to increase the wastewater processing capacity without having to completely upgrade the entire wastewater system. 

About the author   

Rob Tomilson is the Product Line Manager for ENCON Evaporators. He has held various roles during his 13 years with ENCON Evaporators as a Sales Engineer, Engineering Manager and Product Line Manager. Prior to ENCON Evaporators Rob was a sales engineer for several water treatment chemical companies with over 30 years of water treatment experience. Rob has a BS in Chemical Engineering from the University of Maine. Rob has been an elected Trustee for the Veazie Sewer District for the past 11 years, serving as board chair for the past 10 years.  Rob can be reached at 603-624-5110 or sales@evaporator.com.