November/December – 2010
Author: Jeremy Badgery-Parker
Microfiltration may offer a practical, cost-effective and reliable means of removing fungal and bacterial pathogens from nutrient solution.
All water used in the greenhouse hydroponic system must be disinfected prior to use. There are a number of disinfection technologies available to industry. Each has advantages and disadvantages. The operational reliability of a treatment system to disinfect water is essential as is the ongoing management task of any disinfection system, that is, its practicality. Importantly, cost (both capital and recurring) is a critical consideration.
The fairly extensive range of disinfection methods have been described and debated in many articles and forums over the years and increasingly in recent months with the ongoing focus on water efficiency and fertiliser costs in Australia. Commonly, only a couple of the main treatment options get attention.
While it seems a lot of work and effort over the years has gone into developing and describing (and selling) methods of disinfection of nutrient solutions for hydroponics, is the story finished or is there a missing chapter that relates to microfiltration?
Filtration offers a direct physical means to remove pathogens from recycled water and subsequently is a simpler management task for many growers. Every grower will already be employing some form of filtration within their production system. The most common reason will be to prevent emitters from blocking. A typical sand filter may remove contaminants down to around 50 – 70 microns in size. A human hair is around 50 microns thick, while a sheet of newspaper is about 70 microns thick.
Fungi and bacteria are the primary pathogens of concern for most growers and can be retained from nutrient solution using microfiltration. Currently, the removal of viruses from nutrient water is not a critical requirement for Australian growers. If this changes, viruses can be removed by adding ultrafiltration (or another appropriate disinfection option) to the water treatment system.
Key advantages of filtration over other disinfection options include that it does not affect the nutrient profile, it is not hindered by low light transmission and it does not require storage and use of chemicals. Nor does microfiltration pose health or environmental risks. Filtration is also a required precursor to all other disinfection options.
To assess the potential of microfiltration as a disinfection option, a complete package system, comprising a Tekleen automatic back-washable 200 micron filter followed by a 2 module Siemens Dizzer Series 420, multibore UF system, was purchased from Siemens Water Technologies. (Note, there are other filtration companies who could also supply suitable equipment.)
The specified requirements were that the unit needed to be able to treat at least 20,000L/day and to screen to a pore size of 0.2 microns. (The Siemens package processes up to 35,000L/day. Systems with larger treatment capacities are available.)
The assessment was based on three criteria – it must work, it must be cost-effective and it must be practical.
The first criterion is that microfiltration must reliably remove the target pathogens from the water being treated. A key fungal pathogen (Fusarium oxysporum) and a key bacterial pathogen (Clavibacter michiganensis – bacterial canker) were used to test the technology with a series of five preliminary trials.
A highly concentrated preparation of each pathogen was inoculated into a water tank and passed through the filtration unit. Control samples of the water both before inoculation and after inoculation were collected prior to filtration. Samples of the filtrate were collected during the filtration period and a final sample was collected from the filtrate tank at the end of the filtration process.
Clavibacter michiganensis was not isolated from any of the filtrate samples, demonstrating very effective control of this pathogen.
In one trial, a single colony of Fusarium was isolated during the sampling process and again in the final filtrate sample. As no Fusarium was found in a second filtration sampling nor was any Clavibacter isolated in this trial, it suggests that the result is likely to be contamination during the sampling process.
In all trials, the pathogens were isolated from the pre-filtered, inoculated water as expected.
While more tests will be conducted over the coming months to ensure confidence in the reliability of this technology, these results demonstrate that microfiltration effectively retains these key pathogens.
Secondly, the disinfection system must be cost-effective. It is important to recognise that the disinfection system is one part of a closed nutrient solution system. The whole system includes a nutrient run-off drainage point, sump and sediment trap, a dirty water storage tank (acting as a second sediment trap), a clean water storage tank, pumps, control systems and prefiltration.
Using an example greenhouse vegetable farm of 5,000 square metres and an annual water use of 8.75 ML, the estimated set-up cost for a closed system, excluding the disinfection unit, is $16,000 plus installation.
Table 1 presents a comparison of the microfiltration unit with some other disinfection systems. These calculations are for a 10-year period for the whole recycling system and include estimates of all operating and maintenance costs as well as routine nutrient analyses. These calculations include water and fertiliser savings (but do not include rain water capture).
Net present value (NPV) is provided as a way to assess the real value of a potential investment. Where NPV is greater than 0, the investment adds value and is therefore considered worthwhile from an economic perspective. Payback period indicates, in current dollars, how long the investment will take to pay for itself. The return on investment figure is equivalent to how many dollars you would save over the 10 years for every dollar you spend in the same period.
Microfiltration is comparable in costs to chemical treatment options but without the potential hazards and without impacting on the nutrient solution. It is important to note that with larger enterprises, both ultraviolet disinfection and thermal disinfection also become positive investments but appear to be uneconomical at current prices for smaller enterprises.
As with all disinfection systems, microfiltration requires routine maintenance, in particular, backwashing and periodic replacement of membranes. A potential problem with microfiltration is that, without regular and routine backwashing, the membranes will block and need to be replaced more frequently. Budgeting to replace membranes annually decreases the NPV to $31,639.83, drops the return on investment slightly to 1.32 and increases the payback period 5.6 years. This would still be considered a good investment. (It is worth noting that investment in microfiltration would still be positive even if membranes had to be replaced three times per year.) Material and labour costs have been included in the analysis for all the systems.
Finally, in order to be feasible for this industry, the disinfection system needs to be practical. Microfiltration does not impact on the nutrient profile so unlike many of the other options, no post-treatment adjustments are required, saving both time and money and avoiding potential complications. In addition, microfiltration does not produce any residues that must be removed or depleted prior to use in order to avoid phytotoxic effects. The most significant problem, however, is blocking of the microfiltration membranes. It is important that sediment traps be installed and maintained within the nutrient collection system and if retrofitted, sump tanks and drain lines must be properly cleaned prior to installing the filter. Trials are currently being undertaken to determine backwashing cycles and membrane performance under greenhouse conditions. With this information, it will be possible to schedule filter maintenance and determine the true practicality of this method of nutrient disinfection.
This preliminary analysis indicates that there is good economic reason for hydroponic greenhouse growers to install a closed hydroponic system and that microfiltration may be a feasible disinfection option. Note, in making any investment decision, it is important to carefully examine all available information and make the best decision for your situation.
About the author
Jeremy Badgery-Parker BScAgr MBA is the Extension Horticulturist for Greenhouse Horticulture with Industry & Investment NSW (I&I NSW). He provides an extension service on all aspects of greenhouse and hydroponic production and is involved in a variety of research and extension projects for the protected cropping industry. Email: firstname.lastname@example.org