Various sewage sludge treatment methods are cost-effective solutions to enriching agricultural lands. Organic matter known as biosolids that are recovered from wastewater and sewage sludge are rich in nitrogen and phosphorus, as well as other valuable micronutrients.
However, to ensure safety and viability of biosolids, they must first undergo various sewage sludge treatment methods. There are three important reasons for employing sewage sludge treatment: to stabilize the organic content and reduce the offensive odour of the sludge, to eliminate the pathogenic organisms found in the sludge (this includes enteric bacteria like salmonellae, the eggs of the human-beef tapeworm, Taenia saginata, potato cyst nematodes and a range of other viruses), and to minimize diseases spread by vectors such as flies and mosquitoes.
Laws have been implemented that effectively banned the use of untreated sludge because of the potential risks to human health. Today, wastewater is required to undergo biological, chemical, or heat treatment, long term storage, or any other processes that would significantly reduce health risks.
These are some sewage sludge treatment methods currently being used to ensure human health safety:
This method is used to separate sludge into liquid and solid parts. Water content is removed by centrifugation, filtration, and/or evaporation to lessen transportation costs or to improve suitability for composting. Centrifugation is the initial step to lower sludge volume followed by filtration or evaporation. Filtration can be done using underdrains in sand drying bed or as a separate mechanical process in a belt filter press. The process does not treat the sludge and it will still contain pathogens and pollutants.
Mesophilic Anaerobic Digestion (MAD)
This involves the digestion of sludge using microorganisms in the absence of oxygen. This process results in biogas composed of half methane and half carbon dioxide. Biogas is considered a valuable source of renewable energy.
First the sludge is mixed in containers that is heated to 35 degrees Celsius for 12 – 14 days so that the digestion process can take place. Then it goes through 4 methods: hydrolysis, acidogenesis, acetogenesis, and methanogenesis. Complex proteins and sugars are broken down and results in simple compounds such as water, carbon dioxide, and methane. After digestion, the liquid may be dewatered so that it can be kept in storage for several months. The dewatered cake must meet pathogen reduction requirements set by authorities. A disadvantage of this treatment process is the long wait time required (up to 30 days) and expensive capital cost.
Pasteurisation Followed by MAD
Sludge is disinfected by heating it for a minimum of 30 minutes at 70 degrees Celsius or a minimum of 4 hours at 55 degrees Celsius. The aim of this process is to physically disintegrate bacteria or remove the pathogen’s ability to cause infections. The process is followed by MAD to guarantee stabilization of organic matter avoiding possible regrowth of harmful organisms.
Thermophilic Aerobic Digestion (TAD)
This process requires sludge to be pasteurized and stabilized for agricultural purposes. The required heat is a minimum temperature of 55 degrees Celsius running for a period of at least 4 hours and a retention period of 7 days. The sludge is aerated to give aerobic microorganisms the ability to oxidize the organic matter. This reaction subsequently produces heat without any external sources leading to a pasteurized byproduct. The only downside is that this is a more expensive technique because of the energy used by the blowers, pumps and motors needed to add oxygen to the process.
In order for wastewater sludge to be stabilized, alkaline materials are added to raise the pH level rendering conditions unfavorable for bacterial growth. The PH level is raised to greater than 12 for a minimum of 72 hours, maintaining a temperature of 52 degrees Celsius. This is achieved by incorporating quicklime while mechanically dewatering the sludge.
This process involves the mixing of sludge with agricultural byproducts such as sawdust, straw, or wood chips. The idea behind it is that the bacteria will metabolize both the sludge and plant material and produce an exothermic reaction raising the temperature and killing any disease-causing bacteria and parasites. To keep the compost properly aerated, bulking materials may be added to allow air to circulate through the sludge. Materials like corn cobs, nut shells, lumber bark, or even biologically inert materials like shredded tires may be used as bulking agents. After the composting process has yielded sufficient pathogen reduction, the bulking agents may be filtered out for reuse.
Thermal drying is used to achieve up to 80% volume reduction and up to 90% weight reduction. Drying systems use different systems for heat transfer such as convection, conduction, and radiation heating. Techniques that mainly use convection for heat transfer are usually referred to as “direct” dryers. In direct heat dryers, hot air/gas goes through a container and comes into direct contact with the particles of the wet materials. The exchange between the hot air and cold wet cake results in the transfer of thermal energy, which causes an increase in wet cake temperature and evaporation of water. The high temperature air/gas can come from almost any source of heat but is usually produced by a gas or oil-fired furnace.
This technique employs the use of direct steam injection to the sludge and subjects it to both heat (130-180 degrees Celsius) and pressure (7 – 10 bar). It destroys pathogens and breaks down the organic matter for easier digestion and biogas production under MAD.
There are several sewage sludge treatment options depending on the volume of sludge, treatment costs and application. Some methods such as composting are more suitable for rural areas while aerobic digestion and mechanical dewatering are better in metropolitan cities.