January/February – 1995
Story Title: The Sewage Solution
Author: Roger Fox
Sewage sludge poses one of the modern world’s biggest disposal problems. But new research is exploring an application for it in horticulture.
In the quest to ‘clean up our act’ in modern society, urban sewage and the way we manage its disposal, has become an important issue. Within Australia, the topic has perhaps been most contentious in Sydney, ever since sewage effluent became implicated in the pollution of that most sacred of sites – the beach!
Attempts to ameliorate the situation, by putting the offending sludge further out to sea, has really only removed the problem from view. The basic question of how we can dispose of sewage sludge (now known as ‘Biosolids’) without polluting the environment, remains to be answered. So now a new, and in some ways obvious, alternative is being examined: re-using the sludge on land rather than dumping it in the sea. While we use animal manures for fertilising and enriching soils, sewage effluent has largely been ignored as an organic nutrient source for most of this century.
At NSW Agriculture’s Biological and Chemical Research Institute, at Rydalmere in Sydney, a research trial sponsored by the Sydney Water Board is looking at the use of composted sludge in vegetable growing. According to the research leader, Dr Geoff Cresswell, the trial is the only one in Australia taking this approach.
“We’re the only research team currently looking at composting the sludge before it’s used,” says Geoff. “We see that there are particular benefits in composting, the most obvious one being that composting kills human pathogens and parasites, which makes compost a much cleaner material than the digested biosolids.”
The composts used in the trials are produced by Australian Native Landscapes, a composting company in outer Sydney. They are a mixture of anaerobic digested biosolids, and conventional compost ingredients such as green waste (leaves, cuttings, etc) and hardwood sawdust. The result is an organic material with good texture and negligible odour. Like all good composts, it is also easy to incorporate into the soil and appears to have an immediate benefit on the soil’s physical properties.
The compost trial project began 2 years ago with a series of glasshouse experiments, and was subsequently moved out into the field in February 1994. To date, plants in all the trials have shown a significant and immediate growth response to the compost application. But what happens further down the line, and what effects there are on food purity, are the questions the research is seeking to answer.
From the plants’ point of view, one potential problem common to most composted materials is a temporary lock-up of nutrients, particularly nitrogen, in the soil.
“When you make a compost,” Geoff explains, organic wastes, high in carbon and low in nitrogen are generally used. When this is added to soil, the microflora in the soil will start to break it down, using it as a carbon food source. But to do that, the micro-organisms require some nutrients, they need nitrogen in particular, and so they actually compete with the crop for nitrogen.
“So after that first nutrient flush when most of the soluble nitrogen has washed out of the mix, it’s possible in subsequent years that there may be problems with what is known as ‘nitrogen immobilisation’. But there doesn’t appear to be any sign of that in the current studies, so our fears may not be justified.”
But there are, of course, more specific problems that arise in using biosolid-derived products on food crops. Heavy metal contamination is of major concern and the trials have identified the most problematic metals to be copper, zinc and cadmium. These metals do not come from domestic sewage – their presence in sewage is largely as a result of industrial waste.
There are two aspects to the problem of heavy metal contamination. One is its effects on the growth of the plants and the other, more important aspect, is its effects on human health. The preliminary glasshouse experiments, conducted at the Institute, indicated that plant health was not in jeopardy. Now in running the field experiments, plant health is no longer a concern – the emphasis is on human health. Here again however, results have been highly favourable, with no excessive heavy metal levels found in the edible portions of any of the crops tested.
The vegetables being used in the trials are lettuce, beetroot and eggplant, representing a leaf, root and fruit crop. As a comparison, control plants are being grown in untreated soil, and fed with a standard rate of inorganic NPK fertiliser.
To research the impact of using polluted biosolids, one of the composts was ‘spiked’ with heavy metals from a contaminated sludge. The research team were interested to see just what sort of levels were necessary in the sewage compost, before metal levels in the plant became unacceptable. But even here, the only metal that came anywhere near the recommended limit for human consumption, was cadmium.
“So even in the spiked one we had difficulty exceeding the limits,” Geoff explains. “But in what we might call the ‘clean biosolid’ composts, the heavy metal levels came nowhere near the limit.”
“In fact, as it turned out, the cadmium levels, which were our main concern, were actually lower in produce from some composted sludge treatments than they were in the inorganic fertiliser treatment.”
Interestingly, the growth stimulation in plants after treatment with composted sludge, seems to go well beyond what one might explain from nutrition alone. It has been suggested that this response might be some form of biological stimulation at work, involving organic compounds that are absorbed by the plant roots and which directly stimulate growth. A simpler explanation however, is that in improving the structure of the soil, the compost has significantly improved the availability of moisture and air to plant roots.
“My feeling is that it’s a bit like hydroponics,” Geoff explains, “in that plants grow better, because water is no longer a major limitation. Water is very often the most limiting factor to crops in the soil. Adding compost has made the soil more like a potting mix. We’ve opened it right up and in doing that, we’ve increased the proportion of useful soil pores. The water that’s held in these large pores is much more readily available to the plant.
“It appears as though the total amount of water that’s held by the soil is not significantly affected by the compost, but the water is more readily available to the plant. So moisture stress is lessened.”
In addition to the trials themselves, Geoff and his colleague Dr Peter Fahy have been involved in overseeing the compost manufacturing process at ANL. This project has trialled different types of compost, and monitored a range of chemical and physical conditions, including moisture and temperature gradients through the pile, with a view to establishing quality standards.
Ultimately, Geoff sees a potential for the sludge composts to be used in a sort of organic variant on hydroponics. The medium, he believes, could be used in bags to grow crops such cut flowers and perhaps tomatoes and cucumbers, similar to the way in which bags of sawdust are used for crop growing. The major difference of course, would be that the sewage compost would require irrigation with water, but little if any fertiliser. The spent material would pose no disposal problems, since it can be readily added to soil, unlike inorganic media.
Field Trial Results
In the initial field trial in February 1994, all the crops treated with the compost outperformed those receiving only inorganic fertilisers. In the case of beetroot, the yields were up to 30% higher.
Crops in one treatment developed symptoms of nitrogen deficiency, suggesting that available N from the compost may become limiting after 1-3 months. Use of raw sawdust as a bulking agent in this compost may be responsible for this effect. As already discussed, heavy metal residues in the edible portions of the crops were within acceptable levels.
The initial rate of compost application was 125 dry tonnes per hectare. Doubling this rate however, did not markedly improve crop growth. Now that its effects have been proved, the research will look at what the minimum application rate of the compost is, to still achieve the growth benefits.
One of the field trials at Rydalmere, is looking at the effect of different rates of application of the compost on yield and heavy metal accumulation by the crop. There are two control plots, one with inorganic fertilisers applied at optimum rates and one with no additional fertilisers. The compost rates are 100, 50 and 25 dry tonnes per hectare.
Another aspect of the current field trials (by Len Tesoriero) is to examine the disease suppression qualities of composted sludge. There are known benefits in adding organic matter to soil in terms of disease suppression in plants, and according to Geoff the effect is largely a competitive one.
“If you encourage the development of micro-organisms in the soil which are not damaging to the crop,” he says, “then they’re going to compete with the ones that are. When there is such a variety of organisms all competing for the same food, the pathogens don’t necessarily get the upper hand.”
The length of time for which the compost remains active is another important issue, which the trials hope to answer. What, for example, is the residual effect of compost applied 6 months ago?
“I’m hoping that there will be additional benefits,” Geoff says, “other than that initial flush of nutrients we have observed.”
To answer the questions of longer term use, some of the treatments have had compost added yearly, some bi-yearly. A major concern is, once again, what happens to the heavy metal levels over time.
“We want to know what happens when compost is applied regularly to the same soil,” Geoff says. “If you did it every year, for example, would heavy metals eventually build up to levels which could become a problem, even though they may not be present at high levels in the compost.”
The relationship between soil and biosolid-based composts will take time to understand, and best results may be achieved by using a combination of both compost and inorganic fertilisers. In terms of frequency of use, an application every three or four years may prove to be adequate. The resulting build-up of organic matter in the soil, could in turn reduce farmer dependence on inorganic fertilisers.
The results of the trials have already confirmed the usefulness of composted biosolids in horticulture. The hope is that they might also point the way to solving one of our worst waste disposal problems.