On the edge of Hallsta and Braviken paper mills there are metre-high mounds consisting of fibre sludge and biosludge. Each week the mill workers add about another 1 000 tonnes of sludge. To introduce oxygen – a necessity in the composting process – the heaps are constantly turned. After eight to ten months the sludge, consisting of residual products from the biological water treatment and fibre from the process wastewater, are converted to fibre soil reinforcement. This is then sold on to soil producers who mix the fibre soil reinforcement into construction soil or planting soil.