Indian cities generate about 29000 million liters of domestic sewage every day according to the pollution control board estimates. There is no accurate data available on the total sludge generation, but considering that 2% of the total sewage will be converted to sludge, approximately 580 million litres of sludge can potentially be generated if all the sewage is treated. While treated sludge can be used as a fertilizer, untreated sludge can cause severe damage to soil, groundwater and toxicity to plants. Currently, there are no legislations for treating the waste activated sludge and it is mostly applied to agricultural lands without any treatment. This is causing an irreversible damage to the environment and needs urgent attention.
What is waste activated sludge?
Activated sludge is the most popular technique used for treating municipal waste water. During this process, microorganisms degrade the organic material in waste water under aerobic conditions and grow. The excess biomass (microorganisms) produced are separated from treated water in a clarifier tank and sent for disposal.
There are several options available for treating sludge such as composting, anaerobic digestion, incineration and land filling. The most common method in India is drying sludge in open beds as shown in the below image and is applied directly on fields after dewatering. This method requires lot of space which are often difficult to find in STP locations, they produce foul odour and is difficult to handle during rainy seasons. Manual handling is a potential biohazard because sludge contains pathogenic microbes.
Fig: sludge drying beds, India
I started looking out for viable treatment options across the world more than a year ago and was convinced soon that sludge greenhouse is a good option. It’s a simple greenhouse that traps solar radiation and heats up the area inside. The shortwave solar radiation enters the greenhouse and is reflected from the ground as long wavelength. This cannot escape the greenhouse and is trapped inside. Depending on the solar energy, humidity and temperature at the plant location, the amount of air that needs to be drawn through the plant can be calculated using certain drying functions. The water is evaporated from the raw sludge and since its density is lesser than dry air, it rises up and is transported across the greenhouse to the outlet due to suction pressure established by the ventilators. The raw sludge containing over 90% water is spread on a concrete floor inside the greenhouse and is occasionally mixed using a turner. The output is a dried sludge and the moisture content of the final product can be controlled with the operations. The entire operation is automated and hence eliminates the need for manual handling which is biohazard. In few plants in Germany, for example, the water content is reduced to less than 20% and this is sent to incineration plant as a fuel. Since this is a closed system, rainfall doesn’t hamper the treatment process and the long retention time is enough the control the odour. The floor space required is about half of what is required for open drying and the drying rate is faster by more than 100%. Hannover in Germany has just over 1501 hours (62.5 days) of sunshine in a year as compared to over 2400 hours in south India and I was quite surprised to see them implemented widely under despite such weather conditions. Here are some pictures of a sludge greenhouse by Thermo Systems outside Hannover in North Germany –
While this is an extremely efficient technique for treating waste activated sludge, the capital cost can be expensive in the Indian scenario. However, a community plant with a minimum capacity of 5 TPD can be economically feasible. While the structure and the equipment are easy to manufacture the engineering design is complex and needs to be highly accurate.
Traditionally, across the world, waste activated sludge has been used as a fertilizer due to its organic nature. However, in the past decade, many studies have proved that untreated sludge can be extremely hazardous and many countries in the world have responded swiftly by either banning the use of it as fertilizer or by introducing stringent rules for use of treated sludge on fields. Since India is an agricultural economy, this issue is even more significant. We have been polluting the soil and water due the misconceptions about sludge for several decades and it’s time to act quickly.