THE DISEASE RISK IN CLOSED RUN-OFF RECYCLING SYSTEMS
All the European literature suggests that closed run-off recycling systems have such a high disease risk that recycling should not be considered without disinfection of the run-off before its re-use. My experience with closed systems recycling run-off systems in New Zealand and Australia is that disinfection is not necessary if appropriate hygiene precautions are taken.
- The fresh water used must be be pathogen free and preferably disinfected before use. Roof water stored in tanks or dams, any sort of surface water, piped potable drinking water, and water from wells and shallow bores must all be disinfected. Water taken directly (without storage) from deep bores may be safe enough to use without disinfection. Water disinfection is discussed elsewhere in GHVI
- Planting material must be disease free. Rockwool cubes are often the preferred propagating media for plants because they are sterile and delivered to the user in clean packaging,but nursery practices during propagating must follow strict hygiene procedures to ensure the young plants remain disease free.
- Hygiene must be strict in the greenhouse environment. Workers or visitors may easily and inadvertently introduce pathogens into the greenhouse. Foot baths are essential and special clothing (overalls) to wear in the greenhouse are desirable. The greenhouse should have been built so that outside surface water can never flood or get into the greenhouse, this is one of the commonest sources of disease in older NZ greenhouses.
- Microflora which can protect against pathogens has been shown to develop rapidly in recirculating nutrient solutions and can be supplemented by added biological control agents like Trichoderma.
- Avoid plant stress. Stress can be a major factor increasing crop susceptibility to pathogens. Power failures resulting in water stress are a major risk if a standby generator is not available.
There is more relevant information in the GHVI page on Root Diseases in Hydroponics.
RUN-OFF COLLECTION SYSTEM
Run-off for nutrient solution recycling must be collected cleanly and hygienically. The run-off solution, like nutrient solution will be mildly corrosive, and must be protected against metal contamination by using only plastic pipe, polythene tanks and stainless steel or plastic pumps. A very simple but effective low cost system for collecting run-off is described here.
The greenhouse floor must be accurately graded with a small slope over its length. All modern greenhouses are built with a fall, so that the gutters between the multibay roofs drain to the end of the greenhouse. Shorter greenhouse may have a fall from one end to the other end, long greenhouses are often high at the centre with a fall to both ends. The greenhouse floor needs to be accurately graded to the same fall as the greenhouse, preferably by using a laser guided levelling equipment.
The greenhouse floor should be compacted after grading and then completely covered by either black and white polythene mulch (panda) film or white wooven weed mat over a polythene film moisture barrier. The white floor surface reflects light transmitted through the crop canopy back up and into the canopy and can increase total photosynthesis by the crop. The moisture barrier of the film prevents evaporation from the underlying soil and helps to limit the humidity and disease incidence. It also means that any water on the surface of the mulch cannot drain through, and will form puddles, but puddle and pond formation can be limited by accurate floor levelling.
Slabs of expanded polystryrene are laid in lines on top of the floor film at the row positions. The slabs are usually 40 mm thick. The slab width depends on what soilless media is to be used. Slabs 150 mm wide are used for PB18 planter bags, polythene grow-tubes of sawdust or pumice or 150 mm wide coir or rockwool slabs. Coir or rockwool slabs 200 mm wide require 200 mm polystyrene slabs.
Black and white (panda) polythene gully film, 600 mm wide is laid over the polystyrene slabs, before standing out the the planter bags, grow tubes, coir or rockwool slabs onto of the gully film. The edges of the gully fim are then folded up and fastened to planter bags or polythene covers of the coir or rockwool slabs with nylon tags (15 or 20 mm long Dennison tags are very suitable). This forms run-off collecting channels about 50 mm wide and 40 mm deep on each side of each row plants. Run-off drains into these channels and flows to the low end. The channels are closed and so the run-off nutrient solution flows in the dark and does not become contaminated by algal growth, and is protected from leaf, flower or disease debris falling from the crop above.
The low end of the gullies discharge into collecting pipes running across the width of the greenhouse. These pipes are often 90mm diameter PVC stormwater pipe. Tees are set in the pipe at the row spacing. Inlet grill fittings are available for this pipe and are used to secure the gully tails in the tees. This method provides a secure fixing which prevents any surface water entering the collecting pipe.
Each collecting pipe discharges into a small buried sump, often a 400 litre horizontal polythene tank, fitted with a float switch and pump to return the run-off for recycling.
A much more expensive alternative to this collection system is to use hanging gutters. Their advantage is that they can be hung from the greenhouse structure or supported above ground on special legs, but in either case adjustment to obtain a steady fall for drainage is simple. They are also used when double cropping and interplanting to allow an old crop to be ripening fruit below the raised gutters while new plants are growing in good light above the gutters.
SIMPLE CLOSED SYSTEM
The simplest nutrient recycling system consist of a drain water collection system as described in the paragraph above plus a large above ground polythene tank to hold the bulk nutrient solution and the usual trickle irrigation system to apply the nutrient to the crop. The run-off is collected in a 450 litres buried polythene sump. A float switch in this sump is set to start the return water pump when the sump tank is nearly full and to stop the pump when there is about 100 litres of run-off left in the bottom of the tank.The drain water pump delivers the run-off through a polythene pipe to the large above ground polythene tank. A water meter is set in this polythene pipe adjacent to the large tank to record the volume of run-off returned. A Jobe valve or full flow ballcock is installed within the large tank to add fresh water whenever the volume of solution in the tank falls below about four fifths of the tank capacity. This leaves adequate space for plenty of run-off. There is another water meter in the fresh water supply pipe. Nutrient solution is drawn from the bottom of the tank by the main irrigation pump, and delivered to the crop through usual solenoid valves and trickle system under the control of the irrigation controller. This pump can recirculate solution back into the tank via a normally open solenoid valve and a sparge pipe across the top of the tank between irrigation cycles. This ensures that the bulk solution, fresh water and returned run-off solution are kept thoroughly mixed at all times and well aerated. The normally open solenoid valve closes when irrigation is in progress.
The solution concentration, conductivity and pH for this simple system can be manually checked and when addition of stock solution or acid is required, then measured quantities of A & B stock solution or acid can be tipped into the in ground sump, and will be pumped into the main tank with the next lot of run-off solution.
This simple system allows growers using a run-to-waste system to convert to a closed system simply by adding a small sump tank and pump and a large solution tank and to continue using their existing solar integrator and irrigation control system. Manual dosing of the stock solution requires that the crop area be relatively small and the solution tank volume relatively large if the conductivity range is to be kept small. The difference between two readings of the fresh water meter on successive days shows how much water the crop has evaporated, while the drain water meter show what the daily volume of run-off is. The amount of irrigation applied is the sum of the fresh water and drain water and the percent drain water is run-off volume times 100 divided by irrigation volume. The irrigation dose not need to run at night,b should run for some time between irrigation cycles to nutrient solution , run off and fresh water properly mixed.
SIMPLE SYSTEM WITH AUTOMATIC CONDUCTIVITY & pH CONTROL
Solution conductivity and pH can easily be controlled in this simple system by adding a a sampling pot, and an ordinary NFT solution controller like the Autogrow Nutridose. A small bore sample pipe can take solution from the pipe between the irrigation pump and main tank and deliver solution to a sample cup containing the conductivity probe and pH electrode. A 4 or 5 mm bore pipe is adequate provided that it is used with small volume sample cup (200-250 ml is big enough). The sample cup should discharge through an overflow, it must never be allowed to drain dry or the sensor may read low and overdose with stock solution. The overflow from the sample cup should be returned with drain water, either directly into the drain water sump or into the nearest gully or collecting pipe. A & B stock tanks and an acid tank can be set up near the main solution tank and discharge via peristaltic pumps directly into the main tank. A particular advantage of the Autogrow Nutridose controller is that the length of the dosing time can be set as well as an interval time between doses. This allows stock solution addition rates to be matched to the mixing response time of the bulk solution in the large tank.
Polythene tanks with a capacity of 25 m3 are economically priced and popular with growers and can be used with automatic conductivity and pH control on crops of upto about 4000 m2.
SYSTEM WITH FERTILISER MIXING PUMPS
If computer controlled mixing pumps are included in the system then the main solution tank can receive run off and fresh water only and the stock solutions can be added to the flow of irrigation water being pumped out of the main tank, with the computer control sensing the conductivity and pH of the flow and managing the injection rate to precisely control the set conductivity and pH of the solution delivered to the trickle system.
CLOSED SYSTEMS WITH RUN-OFF SOLUTION DISINFECTION.
The flow of run-off is intermittent and it is usual to collect a some volume of run-off in a tank and when the tank is full to pump the run-off through the disinfection device and into another holding tank.
The same disinfection device is used to disinfect fresh water which then needs storing in another tank.
The computerised fertiliser mixing system simultaneously pumps disinfected run-off from one tank and clean water from another tank and blends the two in a set proportion before injecting stock solutions and/or acid into the flow to the irrigation system. The whole process of tank filling and emptying and disinfection followed by blending and mixing requires computer control and coordination with the greenhouse environmental control system, and hence is only seen only larger greenhouses.