Sludge, a semi-solid slurry, is produced by a wide variety of industrial processes, including oil and gas drilling, and mining operations, as well as a by-product of water treatment, wastewater treatment or other types of sanitation systems. Sludge also occurs naturally in some surroundings, notably semi-stagnant marsh areas and farm stock ponds or fishing ponds.
It can be liquid, depending on the source, and it can contain varying proportions of solid particles – often significant quantities of organic waste and biochemical contamination that must be handled carefully in order to protect water supplies and the natural environment from contamination.
In agricultural communities, such sludge is sometimes known as pond muck, and it can be a mixture of dead weeds, fallen leaves, algae, fish and waterfowl waste and the waste from other animals that use the pond. As the layer of sludge builds on the bottom of a pond or shallow marsh, not only does the water depth decrease, but pond health is endangered, or the character of the environment can be irrevocably altered over time. Dredging and shoreline scraping are common deterrents but disposing of the pond muck – or drying the sludge so that it can be used for other purposes – is often problematic.
Treating and disposing of industrial and sewage sludge is an increasingly complicated but vital topic that must be addressed by local communities and national governments as the world’s population grows. Throughout history, and until about the middle of the 20th Century, even in the United States, wastewater (or sewage) was typically discharged into rivers and streams, or directly into the world’s oceans, with almost no treatment. Today, the degradation of water quality and the effects of such pollution on native populations of fish and surrounding ecosystems is a cause of worldwide concern.