by Søren Andersen and Sergey Pigarev
Sewage sludge is rich in nutrients such as nitrogen and phosphorous. It also contains valuable organic matter, useful for remediation of depleted or eroded soils. This is why untreated sludge has been used for many years as a soil fertilizer and for enhancing the organic matter of soil.
While in general terms the market continues to welcome new, effective and affordable sources of soil improver, and the waste industry continues to develop new by-products from waste treatment, it is true to say that there has been a mixed reaction to this application for treated sewage sludge.
Taking Denmark as a case in point, disposal of sludge from municipal wastewater treatment plants in Denmark is regulated by the order of the Ministry of Environment – this legislation permits use of treated sewage sludge as a fertilizer, but more and more farmers are declining to use treated (stabilized and hygienic) sludge on their land.
What are the concerns?
A key concern is that treatment of sludge tends to concentrate heavy metals, poorly biodegradable trace organic compounds, and potentially pathogenic organisms (viruses, bacteria and the like) present in wastewaters. These materials can pose a serious threat to the environment.
When deposited in soils, heavy metals are passed through the food chain, first entering crops, then animals that feed on the crops and eventually human beings, to whom they appear to be highly toxic. In addition they also leach from soils, getting into groundwater and further spreading contamination in an uncontrolled manner.
How can these concerns be overcome?
One company that has been actively seeking a solution to overcome these concerns is Danish firm BioCorrection A/S. It has developed a technology based on oxidative-hydrolytic-destruction.
 An OHD plant can process a variety of organic wastes to produce a liquid oxidate and a solid residue. There is a developing market for both of these by-products |
Essentially, oxidative-hydrolytic-destruction (OHD) is a kind of wet-air oxidation technology (WAO) based on an approach patented in the 1930s and developed in the mid-1960s to reduce the ecological impact of wastewater treatment facilities in the Russian Federation. It involves the oxidation of dissolved or suspended components in water (using oxygen as the oxidizer). Oxidation reactions occur in superheated water at a temperature above the normal boiling point of water (100°C), but below the critical point (374°C).
Wet oxidation has been applied commercially for around 60 years, predominantly for treating wastewater, but also in the treatment of caustic soda, wastes from oil refineries, pharmaceutical industry wastes and other toxic wastes. Usually the by-products of the WAO technologies require post-purification to remove remaining toxic substances, such as heavy metals. With this in mind WAO can be considered the first step in the chain of disposal measures.
Combining oxidation with hydrolysis
BioCorrection has taken further steps by applying WAO in conjunction with alkaline hydrolysis. Alkaline hydrolysis technologies have been extensively used in the chemical industry (for example in wood processing) and in the waste processing industry (for destruction of fats, proteins and carbohydrates). And, having combined its knowledge from various fields – in particular chemistry, mechanics and biology – BioCorrection has identified what it regards as an ideal mix of temperature, pressure and processing time. This ‘ideal mix’ has made it possible to facilitate the conversion of waste into products with commercial potential.
Today BioCorrection is looking to introduce its proprietary OHD technology to the European and American markets, aiming to transform various organic wastes (animal farm wastes, industrial and municipal wastes) into two main by-products:
- a solution of humic substances (a liquid oxidate)
- a solid residue.
To reiterate, this transformation is achieved at high temperature and pressure, thus ensuring full decomposition of organic wastes and chelating of initial pollutants in the resulting products.
The process in more detail
Much like a standard wet-air oxidation plant, the OHD plant includes an input tank and a pressurized tank, otherwise known as a reactor, in which the main reactions occur (see also Figure 1). The input tank is used to prepare the feedstock pulp, which contains a controlled mixture of organic waste (with 5%–15% dry matter) and hydroxide. Conditions inside the pressurized tank are carefully controlled – at a pressure of above 20 bars and a temperature of over 180°C for several hours. Oxygen is also present to ensure successful oxidation of the material.
In these conditions, compounds present in the initial waste (mono- and disaccharides, proteins and fats) are exposed to oxidative destruction – either complete, with formation of CO2 and water, or partial with formation of monomers. Other organic compounds that are more difficult to hydrolyze, such as lignin or cellulose, are also partially oxidized in the course of the OHD process and form the skeleton of the OHD by-products.
A sedimentation process takes place (in separate tanks) during which the heavier fraction, containing heavy metals (lead, zinc, cadmium, mercury) precipitates and is thereby separated from the lighter fraction. This lighter fraction is called the oxidate or humic substance. The heavier fraction is the unoxidized residue.
This physical process does not achieve complete separation of the oxidized and unoxidized parts, but the chelating properties of the oxidate (humic substance) mean that any traces of heavy metals in the unoxidized residue that remains mixed with the oxidate are bound in a stable form and therefore cannot be released by leaching.
A new facility in Denmark
The first OHD unit in Scandinavia is about to be established in co-operation with the Danish Municipality Guldborgsund with a capacity of 20,000 metric tonnes of sewage sludge a year in the first year of operation, aiming at covering the total amount of sludge produced by the municipality by 2009. Guldborgsund is the strait between the Danish islands of Lolland and Falster that connects Smålandsfarvandet in the north with Bay of Mecklenburg in the south.
Although construction of the facility at Guldborgsund has only just begun, the site expects to be operational later this year. And the liquid product (humic substance) is already being introduced to the target market as a fertlizer. BioCorrection has informed municipalities in Denmark about its technology by direct mail and by being an active participant at a number of seminars and meetings. Developing what it regards as the world’s first conventional OHD plant in Guldborgsund is an exciting prospect. BioCorrection is also co-operating with the authorities and with agricultural associations to enable the Guldborgsund project to be used to demonstrate awareness of this technology over a wider area.
Using this OHD technology, BioCorrection intends to reduce the amount of sludge that ends up in landfills after processing at wastewater treatment plants. And it intends to treat both newly produced sludge and existing sludge deposits, which constitute millions of tonnes and occupy large areas in Denmark and elsewhere. In this way the company hopes to reduce emission of volatile organic compounds (VOCs) from what might be thought of as more conventional disposal strategies.
Securing the market
The key to a successful future in this sector appears to lie with successful marketing of the treatment by-products. And, in turn, this depends principally upon assuaging stakeholder concerns about the levels of pollutants in the soil improver (humic substance). When addressing this issue, BioCorrection is confident in the potential of its OHD process to ensure that the majority of heavy metals are bound to the residue and are not present within the humic substances.
Its studies using the by-products have had positive results and the company is now focused on communication. As one example of this, BioCorrection is sponsoring and participating in the 14th International Humic Substances Society (IHSS) Conference, which involves scientists, farmers and other producers of humic substances.
BioCorrection has several applications for the solid residue too. During the initial phase of the Guldborgsund project, the solid residue will not be promoted for use outside the company; however, BioCorrection is developing an application for this by-product as a biofilter for controlling emissions from filling tanks and other evacuating air streams.
There is also potential for using this material in the construction industry as a filling agent for concrete. Research suggests that the composition of the residue locks metals within the material, thus preventing their escape and any subsequent negative effect on the environment.
Trials conducted on BioCorrection’s technology have been encouraging, and research is continuing in this field. The company is running local and international projects in co-operation with leading European and Russian research centres including the University of Copenhagen, and the State Universities of Moscow and Saint Petersburg – thus maintaining links to Russia that reflect the origins of this technology.
A growing move toward sectoral integration
While the market is becoming more demanding in terms of the products used in soil improvement, there is also clear evidence to support a growing sectoral integration. Exemplifying this, BioCorrection is looking at various models for co-operation. It aims to become a preferred partner for the agricultural sector, industry and the public authorities in their quests for solutions to the environmental challenges resulting from organic wastes produced by farms, sewage treatment plants, the pharmaceutical sector and society at large.
‘Our clients will be offered various solutions, depending on their actual needs’ comments Stig Løefberg, Chairman of BioCorrection. ‘Since the beginning of our operations, the focus has considerably shifted to problems related to climate change. This has necessitated development of holistic cross-sectoral solutions for waste management. Wastes that have been used on farmlands up to now will no longer be accepted, unless they are changed to comply with standards set, not only by government, but also by other market players’.
Søren Andersen is Chief Operating Officer and Sergey Pigarev is Project Co-ordinator for BioCorrection A/S.
Phone: +45 3246 0371
e-mail: san@biocorrection.com
