Sewage Sludge: From Problem to Promise

By Glenn Ashton · 17 May 2008

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How to restore the soil - crop - animal - waste nutrient cycle (and save the world.)

The rather unsavoury issue of how we deal with the end products of our sewage has again reared its ugly head in Cape Town. The agricultural smallholding areas of Philadelphia, to the north of the city, are affected by a serious fly outbreak caused by the spreading of raw sewage sludge on nearby agricultural fields.

Sewage sludge is the semi-solid remnant that remains after the rest of the sewage has been treated and returned to the environment. That this water should be reused is not the topic of this article but should be noted all the same.

Untreated sewage sludge is not nice stuff. It can contain pathogens, bio-available heavy metals and organic pollutants, to mention just a few contaminants. And of course it stinks like, well, shit. Flies love the stuff, just as they love most stinky things.

The practice of spreading muck, as it prefers to be called, is practised globally. The consensus is that this is a poor way of ridding ourselves of this material. It continues simply because it is cheap, through the avoidance of further processing and treatment costs for municipalities or utilities. The stuff is given away to farmers, saving the city or its contractors landfilling it or converting it into safer, more useful sources of plant nutrients.

To maximise the benefits of using sludge in agriculture it should be mixed with organic material and properly and adequately composted before being applied to the land. This enhances the nutritive benefits while minimising the risks of contamination and pollution. Composting significantly neutralises contaminants by rendering them less bio-available. Heavy metals are reduced and organic products are largely broken down. By the judicious addition of other substances such as clay, these can be further neutralised.

Most importantly properly composted sludge does not smell of much beside good rich soil. It should attract no more flies than does regular compost. 

Another angle to this approach is that it will diminish the risks of nutrient leaching to watercourses or groundwater. Organic plant matter slowly releases nutrients while retaining water. This is an extremely important consideration in South Africa, where extensive eutrophication of rivers has occurred because of the rapid migration of nutrient loads from chemical based agricultural fertilisers into watercourses. If water retention time in soil is maximised this problem decreases. This is best managed through the addition of organic material.

Given the difficulty of disposing sewage sludge, which constitutes around 0.02% of sewage by volume, there is a clear and present need to have dedicated facilities to treat this material. Just as cities and even small towns produce sludge, there is usually a surplus of garden and other organic plant waste with which it can be mixed to make good quality compost. The microbes present in sewage sludge accelerate and expedite composting of organic material. Of course this is all tied to good waste management practices, another aspect of environmental management that needs close attention.

It is important to remember that the price of oil is directly related to the cost of fossil fuel based fertilisers, presently the most widely used variety. Fertiliser costs have doubled over the past year or so in concert with oil. This directly affects our food costs.

Fossil fuel based NPK fertilisers also have serious negative effects on the microbial activity in the soil, which they actively suppress, disrupt and contaminate. On the other hand compost based fertilisers are not only more available to plants but they also serve to unlock other nutrients already in the soil through supporting microbial activity. A healthy soil is a living soil.

So, a strong argument can be made that using sewage sludge would assist in maintaining healthier soil and hence provide healthier food. After all, just as we are what we eat, the same applies to plants.

We have seen the nutrient value of food crops drop sharply around the world, especially since chemical based fertilisers and pesticides arrived with the herald angels of the green revolution. This problem can be reduced through compost application as a far higher variety of nutrients and micronutrients are made available, not just NPK. A shift away from artificial to more natural fertilisers is therefore urgently needed for health, economic and as ecological reasons.

Just as a healthy body needs less attention, so healthy soil with high levels of biological activity is more resistant to diseases.

One of the main point sources of pollution, especially water based, is through high nutrient loads. These mainly come from agricultural fertilisers but also emanate from poorly run and obsolescent sewage treatment systems around the country, which have an increasingly poor record of complying with basic standards. Victor Munnik recently wrote an excellent analysis of this.

The other serious point source of nutrient pollution is from intensive livestock farming operations – particularly feedlots and piggeries. But just as these nutrients can pollute, so too can they be used as food for plants. At the moment much of this is unwisely disposed of, causing further problems.

What we need to do is to fix the broken nutrient cycle. We need to tap into this smelly waste and return it to our food production cycle. At the moment we have an open loop where the nutrient released from consumption of food by animals is treated as a waste product. I say animals because more food is consumed by livestock than is eaten by people. We need to capture both human and animal waste if we are to close the loop again by shifting this otherwise lost nutrient back into our food supply. This would spur us toward system sustainability.

However we cannot stick our head in the clouds believing this to be the ultimate solution. In order for this to work it must be exceptionally well run and managed. There needs to be batch measurement and mitigation of heavy metals and other noxious contaminants. If this does not happen, the ecosystem is at risk from insidious contamination.

The composting process needs to be run in modern facilities with constant monitoring. If a consistent product is not supplied then farmers will not use it. If it is a health hazard, nobody will.

We must also not allow the control of this process to become alienated from public oversight. History has shown fertiliser manufacturing industries have been used as a route for disposal of toxic waste. Conventional chemical fertilisers around the world have been shown to be routinely contaminated by noxious chemicals and heavy metals in unacceptably high concentrations. It is questionable whether the private sector can be fully trusted to manage this strategically important task in its entirety, given its track record. 

We do however have fairly good state structures that could be adapted to manage this waste cycle. The Department of Water Affairs is the natural home for such a project in South Africa. It probably has the best in-house expertise in this matter. Our large urban municipalities generally have a pretty good handle on the matter even if they do suffer from widespread inadequate capacities. Still, it seems almost criminal to simply pump macerated sewage into the ocean, as occurs in most of our coastal cities. 

Just as bigger municipalities have high sewage loads they also have massive amounts of available garden and organic household waste that is ideal co-composting material. Certain aspects of management of the cycle could be handled by the private sector but tight guidelines must be put in place and strictly applied. If judiciously managed even small municipalities should be able to design in such as system and amortise the cost in a short time. There is need to design ideal systems that can be simply applied.

There are those who say that we should never dispose of sewage sludge into our food chain in any form. I partially agree. These sentiments are mainly directed towards untreated sludge, yet even composted sludge remains problematic for some of the reasons stated above.

But what alternatives have we? The most common way to deal with sludge is to dump it in landfills. This contaminates groundwater and generates high levels of gasses, including greenhouse gasses methane and carbon dioxide. It can also be burned in dried or pelletised form. This releases contaminants and CO2 directly into the atmosphere, not to mention the waste of energy and potentially useful components. Therefore alternative options are poor not only because they all break the nutrient cycle. Those who oppose going this route must at least offer better alternatives.

Given the existing risks to our supply of chemical fertilisers it makes eminent sense to shift towards increased utilisation of this new closed loop nutrient cycle. Besides being ecologically sound, it may also encourage better public awareness of what not to dispose through the drainage system. If people realise they are eating what they are putting down the drain they may prefer to keep it natural.

While going this route has both up- and downsides it would seem that the time is ripe to pursue this as a matter of policy from both sides – from civil society and from the legislative authorities. It also makes eminent economic sense. Sludge and compost equal fertiliser. Given the doubling in price of this commodity, the argument to go this route can be made in purely financial terms. It could certainly be a boon for food security and for small and emerging farmers, particularly if the product is of suitable quality to be used in combination with ecological farming practices.

As in all things this project offers risks and opportunities. It is up to human innovation to balance them while managing the outcomes.

Ashton is a writer and researcher working in civil society. Some of his work can be viewed at Ekogaia - Writing for a Better World. Follow him on Twitter @ekogaia.

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