Comparison of wastewater sludge disposal methods: a guide to make the best decision
The sludge has to be dewatered, dried and reduced to the lowest volume. Keep reading and find out how the process works.
Sludge treatment and its disposal can represent up to 40% of the operational costs of wastewater treatment. Reduce the costs by choosing the most effective sludge treatment method. But which one is it? Read our article comparing all the methods and find out!
Before we begin, let's define the term sludge. Sludge, which is basically a mixture of microorganisms, is a by-product of the biological wastewater treatment.
The microorganisms remove the contamination in the form of organics, nitrogen, and phosphorus. The volume of the sludge increases while the microorganisms reproduce throughout the removal process. As a result, sludge has a slurry structure and increases in volume, requiring a specific treatment.
For the purposes of further treatment, the sludge has to be dewatered, dried, and reduced to the lowest possible volume. Keep reading and find out how the process works!
Aerobic and anaerobic sludge treatment
As you certainly know, both aerobic and anaerobic sludge treatment employs microorganisms and bacteria. Anaerobic bacteria convert organic matter into biogas, reducing the volume of sludge by up to 50%.
Since the biogas is formed from organic matter, the bacteria use the biogas for their growth only to a limited extent. The volume of the sludge produced during anaerobic treatment is thus lower than with the aerobic process.
There are several requirements applied to sludge in order for the anaerobic bacteria to have the expected results. Exact temperature from 35 to 37ºC is required, the sludge can be neither too acid nor too base (pH must be balanced and monitored), the microorganisms need continual food supply, and no toxic substances can enter the system along with the sludge.
Aerobic and anaerobic sludge treatment differs. Anaerobic treatment is performed in the absence of air so the sludge can be reduced in hermetic digesters. For the aerobic process, on the other hand, air is mandatory in order for the sludge to be periodically mixed and oxygenated.
The advantage of aerobic water treatment from a financial perspective is its simplicity, and thus lower staff qualification requirements. However, the aerobic process requires large volume and high consumption of electrical energy for tank aeration.
The anaerobic treatment is slightly more complicated, but its great advantage is the generated biogas to be used for heat and electricity production. This process also comes with certain energy consumption to heat the digesters and maintain the said temperature of 35°C.
Besides the costs consider also the odour which is typical for both treatment methods.
The good news is that if, using the anaerobic method, you process a sufficient volume of sludge and convert it to biogas, the WWTP can achieve energy self-sufficiency, and this, at the end of the day, leads to lower costs.
Solar drying requires sufficient space since the partially dewatered sludge is transported to a drying hall where it is tipped into large beds and left to dry for several days.
Before you decide for this method, consider all its pros and cons. Since it is the radiation that dries the sludge for the most part, the energy costs are minimal. Electrical energy is required only for the ventilation system and for turning the sludge.
On the other hand, don’t forget the large area required for solar drying. If you operate a large WWTP, construction of the greenhouses can be challenging. In such cases, drum dryers can be a more suitable alternative. These might be more energy-intensive, however, they can be operated on much smaller areas.
It is true that while drum dryers evaporate water at the temperature of 100°C, the greenhouse can work at much lower temperature.
The capacity to reduce the sludge threefold also speaks for the benefit of solar drying, i.e. if you spread three tonnes of sludge, it can be reduced to one after it dries out. This allows you to save the costs for transportation and disposal.
As far as the sludge disposal is concerned, dried sludge contains only a minimum water, and can therefore be used to generate electricity and heat or as a soil fertilizer.
Imagine a drum filled with sludge. Valves and pipes are so arranged that, as the drum rotates, a vacuum is applied, drawing out water from the solid mixture.
Furthermore, a mat of sludge is left on the outer surface of the filter and scraped before it enters the tank again.
The efficiency of vacuum filtration depends on the volume of sludge left on the filter and the time required. Sure, there are slower filtration systems and more effective and quicker ones. You should make a decision based on the volume of sludge and how fast you need to treat it.
Also, don’t forget that filtration itself does not dry the sludge fully. This method dewaters the sludge only to 15 to 25%, i.e. the mixture contains 75% of water. Therefore, another drying technology has to be applied to dewater the sludge fully.
Furthermore, vacuum filtration goes with higher operational costs. These can, however, partially return by proper care of the drum and prolonging its life. Clean the drum dryer after every treatment session. Ideally, pick a drum made of the best possible material.
The low space requirements also speak for the benefit of vacuum filtration as it can be used in smaller plants. And since it is situated indoors, the weather conditions do not play any role.
Dried sludge can be incinerated to produce energy.
Sludge fields are the simplest, yet the least effective method of dewatering. Why?
You spend a lot of time building the field consisting of thicker grit boards. The material, however, cannot be fully sealed, since the sludge must be ventilated and requires space for the water to drain.
The drying is not based only on drainage. The water also evaporates. Similar to solar drying, the sludge beds are weather sensitive. When it rains, the water must evaporate again. Therefore, the field can be covered by glass or plexiglass.
Drying the sludge on fields is subject to careful deliberation. Besides slow sludge separation, another factor against sludge fields is the large area and strong odour.
Machine dewatering or vacuum filtration is a more convenient alternative.
Drying and incineration
Unlike drying halls or sludge fields, it is much easier to dry the sludge using hot gas. The sludge is heated to a temperature at which the water is immediately removed from the solids.
This method offers a reduction in sludge volume by a minimum of 50%. The produced mixture can be subsequently incinerated, and thus converted to harmless ashes. If you do not wish to incinerate the sludge, it can be also used as a fertilizer.
In such a case, however, the sludge must undergo an analysis beforehand that shows the presence or absence of heavy metals or other unsuitable substances which would otherwise enter the soil.
Compared to other methods, the machinery for sludge incineration also requires large space. The dryer, along with the incineration room, must be built outside. And don’t forget that you need workers to operate the machines. Ideally, in continuous operation.