 John W. Bartok, Jr. |
Wastewater from greenhouse operations can come from five separate sources: greenhouse roofs, driveways/parking areas, indoor growing benches, outdoor growing beds and flood floors or benches. Best management practices should be used in handling water from each of these locations.
Rainwater
In most operations, the greatest amount of water comes from building roofs. A 1-inch rainfall on an acre of impervious surface such as a greenhouse roof or parking area amounts to about 27,000 gallons of water. Good drainage design is required to handle this water without degrading the water with sediment, pollutants or debris.
Rainwater from greenhouses can be kept relatively clean with grass or stone-lined swales. Directing this water to a detention pond or wetland will allow most sediment to settle out before it reaches a brook or stream.
For gutter-connected greenhouses consideration should be given to installing a rainwater harvesting system to store some of the water for use in irrigation. Removing this water from runoff lessens the impact on nearby wetlands or streams. Storing this water in a heated area tempers it before using it for irrigation.
A basic harvesting system consists of a storage tank, a roof washer, inflow pipes, overflow pipes and a diverter to redirect the excess water when the tank is full. Concrete or plastic tanks can be used but are usually limited to about 15,000 gallons. Corrugated steel tanks can be built to almost any capacity as the tanks are delivered in preformed panels and assembled on site. A retention pond can also be used, but greater filtration is needed due to the potential for greater sedimentation. A roof washer is a device that diverts the first flush of water that is collected that may contain leaves, dust, bugs and bird droppings.
Driveways and parking areas
This space can add up to a significant amount of impervious area if it is paved. There is a greater impact if some of the area is sloped. Non-paved driveways and parking areas should have a minimum 10-inch compacted gravel base with 2 inches of processed gravel on top. This allows for good drainage underneath. Maintaining a cross slope of 3 percent from the middle of the driveway to the edges allows water to flow off to a swale. A curtain drain with 6-inch filter fabric pipe on the uphill side keeps water from getting under the driveway. Where the grade is greater than 10 percent, the driveway should be paved with a minimum of 3 inches of bituminous concrete laid in two courses. This prevents erosion of the driveway.
Truck turn-arounds, dock and material handling equipment areas should have a bituminous paving over a 12-inch minimum granular base. Adequate natural drainage or culverts should be installed to remove runoff. Drainage from paved areas with considerable vehicular traffic or where vehicles are parked should be filtered through a sediment/oil separator to remove sand, silt, oil and growing media before it is discharged to a wetland or brook. For large impervious vehicle areas, it may be desirable to have the water directed to a detention pond for further settling.
Indoor growing areas
Careful selection of the fertilizer and application that meets the nutrient needs as the plants grow can help to reduce environmental impact. With wire mesh benches, leached water drips onto the floor. Experience has shown that with careful watering, there is very little excess water that runs off. Most of the excess water evaporates within the greenhouse.
When benches are not full or when container plants are spaced far apart, significant runoff can occur. If the floor is concrete, drains can be installed to collect and treat this runoff.
Recirculating flood floor and bench systems eliminate runoff as irrigation water is returned to holding tanks. Provision has to be made to dispose of the water from cleaning the holding tanks several times a year.
Outdoor growing areas
Outdoor areas add inexpensive production space especially in late spring when greenhouse space is at a premium. In uncovered areas, the leaching of irrigation water from containers can add significant nutrients and pesticides to the runoff.
Heavy rains can also leach excessive fertilizer and pesticides. In areas that encounter intense storms, placing a vinyl liner with drain tile buried in a few inches of gravel allows runoff to be collected and treated before it reaches a wetland. A holding tank, detention basin or retention pond makes for good storage. Water can be treated for reuse or sent through a constructed wetland.
Constructed wetlands
The use of constructed wetlands has increased in recent years as an effective method of removing pollutants from wastewater. A constructed wetland is fairly simple consisting of a sediment trap, filter bed, wetland and retention pond.
The sediment trap removes the solid matter (growing media, sand, leaves, etc.). A tank or pond can act as a sediment trap. The tank or pond has to be cleaned when the solids build up in the bottom and are disposed of by spreading on agricultural land or by disposing in a landfill.
The waste water is then distributed over a filter bed. This is an area of soil, grass and a vinyl liner. The fine sediment which contains phosphates and nitrogen is removed. The grass is mowed occasionally and the clippings which contain nutrients are taken out of the system. The clippings can be used for compost.
A constructed wetland is a vinyl-lined area with a gravel growing medium that supports water plants. The plants need to be selected for the specific hardiness zone but frequently include water lilies, sedges, cattail and wild rice. The plants remove the remaining nutrients leaving water that is 99 percent clean.
A retention pond can be added to hold the clean water for several days to complete the process. Water from this pond can be released to a stream or drainage area.
All of the above systems require engineering design to handle maximum runoff conditions. Permits are required in most states. Flow rates and nutrient/pesticide levels of the different sources should be monitored on a regular basis to have data available if questions arise by regulatory agencies.
John Bartok Jr. is faculty emeritus, University of Connecticut, Department of Natural Resources Management and Engineering, jbartok@rcn.com.
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