Neiker-Tecnalia develops new types of substrates using sewage sludge and metallurgical waste

Sludge from waste water treatment, ash from biomass combustion, metallurgical waste, and barley straw have been used . The new substrates were found to have some ideal properties for plant growth, e.g. high levels of nutrients (nitrogen, phosphorus and potassium), high acid buffering capacity, and organic matter stability. The research has been carried out by Dr. Fenxia Yao, who has recently presented it as part of the work for her PhD thesis, which constitutes the first PhD thesis in the field of Technosols in Spain .

In horticultural and forestry production systems, the production of container grown plants has undergone spectacular development in recent years owing to the advantage it offers over direct sowing or field crop cultivation. The total volume of crop growing media consumed in the EU is reckoned to be between 20 and 30 million m3 per year, with peat covering 85-90% of market needs. As peat is a valuable, non-renewable organic material, there is great interest in the quest for blends of waste products from urban and industrial processes that can be used to substitute peat, and which at the same time contribute to an effective use of resources.

Neiker-Tecnalia researchers, in collaboration with the University of Santiago de Compostela (Galicia, Spain), have developed new types of Technosols to be used as substrates in the greenhouse cultivation. They were formulated from mixture of sewage sludge, green foundry sand (sand used in the metal foundry industry to produce moulds into which the molten metal is poured), Linz-Donawitz slag (slag from steel refining), barley straw, and biomass combustion ash. Three different types of sludge were employed: anaerobic, aerobic, and lime-treated aerobic sludge. The proportions of each ingredient were: 5% of foundry sand, 10% of LD slag, 2% of barley straw, 23% or 33% of combustion ash, and 60% or 50% of sewage sludge.

The results show that the Technosols elaborated from anaerobic sludge contained a higher quantity of primary nutrients –nitrogen and phosphorus – as well as organic carbon, whereas the Technosols derived from aerobic sludge had the lowest level of organic carbon and macro nutrients. The Technosols formulated from aerobic sludge treated with lime had the lowest availability of phosphorus, due to precipitation of phosphates with calcio.

As regards plant yields, the results show that the highest yields are obtained in the Technosols made from mixtures in which anaerobic sludge was used as the organic component. This is consistent with the availability of the nutrients existing in these Technosols. Furthermore, the bioavailability of heavy metals has been efficiently decreased in the Technosols.

A channel for scientific research

The research was carried out by Dr. Fenxia Yao, who obtained her Bachelor's degree at the Shenyang Agricultural University (China), and her Master's and first Doctor's degree in the Graduate School of Chinese Academy of Sciences (China). Fenxia Yao has become the first person to submit a PhD thesis on Technosols in Spain .This thesis, supervised by Dr. Marta Camps and Dr. Felipe Macías, was defended at the University of Santiago de Compostela on 31 January. It opens up an important channel for the scientific study of Technosols and their applications.

Technosols make a considerable contribution to the environment. Apart from being used as growing substrates, they can be applied to rehabilitate areas degraded by activities to extract minerals, or by public works, etc. At the same time they serve to reuse both organic and mineral materials, which would otherwise end up in landfill sites.