Valorization of sludge from the quartz industry as soil amendment and crop production

Abstract Europe is moving towards a resource efficient economy implementing as initiative the valorization of waste products. Hence, the use of waste sludge from the quartz industry as amendment in agriculture represents a promising alternative to recycle this by-product. The aim of this study was to test the suitability of the sludge from the quartz industry as soil amendment for agriculture. Four samples with different sewage sludge from the quartz industry and a control were investigated in a pot experiment with wheat ( Triticum aestivum L.), barley ( Hordeum vulgare L.) and sunflower ( Helianthus annus L.) cultivated in a sandy soil classified as Inceptisol Ochrept Xerochrept from the Spanish province of Segovia during 2015 and 2016. The following treatments were applied: (1) 25% sewage sludge and 75% soil, (2) 50% sewage sludge and 50% soil, (3) 75% sewage sludge and 25% soil, (4) 100% sewage sludge and (5) 100% soil (control). The concentration of potentially toxic heavy metals was found to be below the critical limits imposed by regulations concerning the agronomic use of sludge. The application of this sludge also resulted in an improvement of soil properties, increasing a maximum of 31% the water content, 0.98% the oxidable carbon and providing a higher concentration of nutrients. In sludge treatments, the CE did not exceed the value of 2 dS cm −1 for sensitive field crops and for the pH the highest value achieved after the sludge application was 8.4 for the 100% quartz sludge treatment, which does not exceed the 8.5 established as limit for sodification risk. In relation to crop growth, the sludge treatment 25% sewage sludge and 75% soil presented positive results, being the height 0.715 m, 0.764 m and 1.16 m for wheat, barley and sunflower, respectively. The nature of this industrial sludge causes some deficiencies in crop yield when compared with the control, being the maximum difference in nitrogen and protein concentration between the sludge treatments and the control of 20%. These results suggest that the application of sludge from the quartz industry can potentially serve as soil amendment. The combination of this sludge with a supplementary nitrogen source could be further studied to improve the crop yield.