In wastewater treatment there are typically two types of contaminants that require removal – Dissolved Solids and Suspended Solids – and both can cause issues in water treatment systems.
Everyone has experienced dissolved solids in the kitchen when adding salt or sugar to cooking. Similarly in wastewater, dissolved solids are dissociated into the water, existing in a homogeneous molecular state alongside the water molecules.
While you cannot see dissolved solid particles, they often cause the water to be tinted an off-colour when held to light.
Dissolved solids (jar on the right) cause the wastewater to be an off-colour. Image: Wikimedia Commons
Treating dissolved solids can be quite challenging because these particles are in the same liquid phase as the water. Some common treatment methods for dissolved solids include using chemical reactions, molecular sieves (eg. reverse osmosis), or evaporation techniques.
Many materials are also highly insoluble in water, and these remain as larger particles called suspended solids. Typical suspended solids in wastewater are dirt or sand particles, biology, chemical precipitations or industrial solids entrapped in the water.
A clarifier removes suspended solids from wastewater on a very large scale. Image: Wikimedia Commons
Suspended solids can cause a range of issues in wastewater treatment systems if left untreated, such as scaling and clogging of pipes and pumps, accumulation in tanks and interference with instrumentation.
The phase difference of suspended solids in wastewater (solid vs liquid) makes their separation and removal much easier and cheaper than treating dissolved solids.
Often a simple piece of equipment such as a settling tank is enough to allow the solids to separate by gravity in the conical section at the bottom of the tank, while treated water overflows at the top.
Settling tanks are a low-cost option for suspended solids removal
Settling tanks can be very large if the wastewater flow is large, however they are very cheap to build, and usually require no chemicals or power to drive the separation.
Filters are another common type of equipment used to treat suspended solids in wastewater, with many types available depending on the form and size of the solids.
All filters use a similar mechanism , pushing the wastewater through a medium which allows liquid to pass but traps the solids. A few of the most common types of filters are listed below:
Bag filters – use a woven fabric to trap the solids
Media filters – use a bed of media (such as sand) to trap the solids
Cartridge filters – use a fibrous polymer material to trap the solids
Disc filters – use discs to trap the solids
Media filters are large, enclosed vessels that contain media (such as sand) to filter the water. Source: Wikimedia Commons
Filters are simple to install and use, however they often require regular maintenance and parts replacement (such as new bags or cartridges), as these parts become clogged with solids.
Once removed from the wastewater, the solids are in sludge form (typically 5 to 10 per cent solids and the rest water), and are often further dewatered to reduce the volume of waste before disposal.
Sludge dewatering is achieved by a range of modern technologies such as geofabric bags, filter presses or centrifuges. More information on sludge dewatering can be found in our article here.