The common question when we are confronted with the idea of biosolids is whether it is safe to be used on land that will grow food or produce. The short answer is yes, but the longer answer will take some time to explain. For one, you need to understand that in order to come up with a clean effluent that can be discharged to watercourses, the wastewater must under go three to four different stages of treatment to remove all harmful contaminants and leave only the most organic of residue.
When the waste is disposed through the wastewater stream, there will be a preliminary treatment stage wherein coarse solids and grit are filtered using screens and other devices. These coarse materials are not going to be incorporated in biosolids and in fact, this preliminary stage is an important step to making sure that the next stage will deal only with materials that can be converted into organic materials.
This stage usually involves gravity sedimentation of screened wastewater to remove settled solids. Almost half of the solids found in wastewater are filtered, screened, and removed through the primary treatment stage. After which, the residual, which is a concentrated suspension, is called primary sludge. No, this is not the same as sewage sludge, which will eventually become biosolids. Primary sludge is the pre-treated stage of sewage sludge before it is treated to become biosolids.
The secondary stage is a biological process in which biodegradable materials are removed. The process uses microorganism to consume the dissolved and suspended organic matter. This will then produce carbon dioxide and other by-products. The organic matter helps in increasing the presence of microorganisms because it contains nutrients from which these organisms feed. When microorganisms feed, their density increases and they began to settle to the bottom of the processing tanks. They are separated from the clarified water. This concentrated suspension is called secondary sludge, biological sludge, waste activated sludge, or trickling filter humus.
Tertiary or advanced
If the effluent needs to be used as direct discharge to a drinking water source, for example, it will need to undergo a tertiary or advanced treatment. The residual that will be collected after the tertiary treatment consists of chemicals that are added to the final effluent. These are reclaimed before the effluent is discharged to the water source. They are not incorporated into biosolids.
The residue from the primary and secondary treatment stages are the most commonly used organic materials for the production of biosolids in the municipal level.