This issue of Waste and Resource Management deals with: an example of the re-use of waste products from the paper manufacturing industry, enhancement of a waste treatment process through the optimisation of different waste streams and a novel means of managing liquid waste from the dairy sector through growing plants in saturated growth media of different types. The three papers are all based on laboratory experimentation, with a broad spread of background perspectives covered in each paper.
Provided the lessons of these laboratory experiments can be then demonstrated in real-world exemplars, they help to progress resource efficiency and the move towards a more circular economy. However, there is a need to ensure that the preparatory requirements for laboratory experimentation can be reasonably replicated without too much manipulation of feedstocks when applied in the real world.
Chagas and Holanda (2022) demonstrated the ‘Potentiality of using paper sludge waste in the manufacture of red wall tiles' by substituting up to 5% of the materials incorporated in the manufacture of these tiles with paper sludge from paper mills. Brazil is the world's third largest producer of these tiles with around 800 million m2 manufactured annually. The paper sludge they used had a high calcite and kaolin content and was therefore ideal to replace the dolomitic limestone component of the materials used. Their experimental application showed that the tiles conformed fully to the physical and structural requirements of the International Organization for Standardization (ISO) standard for ceramic tiles. As paper sludge in most developing and middle-income countries is either dumped or landfilled, its incorporation into these red wall tiles has a huge benefit in reducing waste but there is also a more limited role in terms of substitution of raw material inputs.
Mohammed and Ismail's paper, ‘Green approach for nutrients removal from real dairy wastewater via constructed wetland’ (Mohammed and Ismail, 2022), shows the advantages of using a common robust plant, Canna indica, planted into a variety of different types of growth media to remove polluting substances from dairy waste. Their laboratory-based experimental set-up utilised six containers which were filled with gravel, sand and soil both at 100% and 50:50 mixtures, with a few of the Canna indica plants in each one. The dairy wastewater came from a dairy in Baghdad, Iraq and this was slowly passed through each of the containers over a period of 160 days. During that time the cleaning potential of these treatment options was measured to assess its capability to deal with nutrients, specifically phosphate and nitrates. All the media proved successful in this nutrient removal. The authors also examined the potential advantage of utilising the cleaned water for possible irrigation purposes and again this proved highly successful, albeit again in a laboratory setting with a limited number of wheat seeds germinated.
Kuroshkarim et al. (2022) in their article, ‘Effect of high-carbon co-substrates on anaerobic co-digestion stability and performance’, mainly focused on how sawdust could be used to optimise the output of methane gas from anaerobic digesters (ADs). As with the paper waste examined by Chagas and Holanda (2022), sawdust is a readily available waste product that has a commercial value but not in all locations or from all wood-based manufacturing establishments. This material is particularly useful where the AD bioreactor is used to process high-nitrogen (N)-content biomaterials, such as animal manures. The addition of sawdust wood waste can help raise the carbon (C) proportion to the ideal C/N ratio, which, for optimal performance, is in the range 20–30. However, the wood waste would need some preparation in order to act as a suitable medium for anaerobic degredation and in this experimental example the sawdust was ground to a powder to pass through a 1 mm sieve. While the paper focused on the use of cattle manure, algae and sawdust in varying proportions it provides good cross-referencing to other studies that have utilised animal manures, algae and other sources of carbon. Again, there was the use of small containers with different mixtures of the three materials, which were allowed to biodegrade and produce methane for 30 days to determine the optimum mix of feedstocks for the AD processing.
The potential application of each of these three different waste-related measures will vary. For sawdust there are other more beneficial opportunities for its use, especially as to be of greatest use in an AD plant it would need to be considerably processed before putting it into the AD reactor. The polishing of final effluent from the dairy sector by means of use of suitable plants and growing media is a sound option but does require both reliable monitoring and good maintenance. The greatest opportunity lies with the use of paper mill sludge; indeed, this use of paper sludge for the manufacture of red wall tiles has already been adopted by several paper mills in Brazil as a means of improving their sustainability and reducing their environmental impact.
