Once the sludge is thickened, two options are available for further treatment of the concentrated sludge. It can be dewatered to a solid content of between 30-40% or it can undergo stabilization processes to reduce the organic materials in the sludge before going to the dewatering step. Coarse primary solids and secondary sludge (sometimes called biosolids) accumulated in a wastewater treatment process must be treated before disposal to ensure environmentally responsible and lawful outcome. Sludge is often inadvertently contaminated with toxic organic and inorganic compounds and is nutrient-rich.
The objective of sludge stabilization is multifaceted: it reduces pathogens; it eliminates odor; and it reduces organic matters, preventing or inhibiting future decomposition—this is relevant to sludges from many food-processing wastewaters. Sludges are stabilized to prevent anaerobic breakdown naturally during storage of sludges (a process termed as putrefaction), producing offensive odor. Stabilization of sludge can be done chemically or biologically, though the latter is more common and effective. Lime stabilization is achieved when a sufficient amount of lime is added to the sludge to alter the value of pH to a high level (>11) that no microorganisms can survive. A similar lime treatment can be applied after sludge dewatering to achieve the same objective of chemical stabilization. Because lime treatment does not destroy or alter any organic matter, in order to prevent future decomposition of organic solids in the sludge, an excessive amount of lime is often required to maintain the high pH value.
Biological stabilization utilizes biological (in many cases, microbiological) agents to reduce organic matters in the sludge, a process often termed digestion. There are a variety of digestion techniques, the purpose of which is to reduce, in addition to the amount of organic matter, the number of disease-causing microorganisms present in the solids. The most common treatment options include anaerobic digestion, aerobic digestion, vermistabilization, and composting.
Anaerobic digestion is the most common and widely used sludge stabilization process. Anaerobic digestion also helps reduce global warming. If sludge were landfilled without stabilization, it would still break down naturally (and anaerobically, most likely); however, the biogas (mixture of 55-75% CH4 and others, mainly CO2) would escape directly into the atmosphere, and CH4 is a worse greenhouse gas than CO2. In this way, anaerobic digestion is considered to be a sustainable technology and biogas is considered to be a renewable fuel if utilized.
Aerobic digestion is a process of treating the secondary sludge from the biological wastewater treatment process, such as activated sludge and trickling filters; primary sludge is better treated by anaerobic digestion (see Chapter 4). The secondary sludge is primarily insoluble solids such as biomass of microorganisms. The objective of aerobic digestion is to degrade insoluble solids in an aerobic environment. Aerobic digesters are simply CSTRs, not much different from those used in activated sludge. Both bubbling and mechanical aerators achieve mixing oxygen into the liquid in the tank. By optimizing the oxygen supply, the process can be significantly accelerated.
The aerobic digestion is designed to treat excessive amounts of sludge from the activated sludge process and other biological treatment processes. Early attempts to treat this type of sludge with anaerobic digestion met with little success because of its low solids content and the highly aerobic nature of the sludge. High water content (98-99%) of this type of sludge also prevents economical dewatering by mechanical means without substantial thickening. In small communities, the high capital investment requirement associated with thickening and anaerobic digestion equipment also prohibits the use of anaerobic digestion; these communities are likely to choose aerobic digestion instead.
An aerobic digester normally operates by continuously feeding the raw secondary sludge into the tank punctuated with supernatant and sludge withdrawals. The aerobic digester is aerated continuously while the tank is being filled and for the period right after that. Once aeration is stopped, the solids are allowed to settle by gravity. The supernatant is decanted and a portion of the gravity-settled sludge is withdrawn.
Anaerobic digestion is a bacterial process that is carried out in the absence of oxygen. The process can either be thermophilic digestion, in which sludge is fermented in tanks at a temperature of 55°C, or mesophilic, at a temperature of around 36°C. Though allowing shorter retention time, thus smaller tanks, thermophilic digestion is more expensive in terms of energy consumption for heating the sludge.
Anaerobic digesters have been around for a long time, and they are commonly used for sewage treatment and for managing animal waste. Increasing environmental pressures on waste disposal has increased the use of digestion as a process for reducing waste volumes and generating useful by-products. It is a fairly simple process that can greatly reduce the amount of organic matter that might otherwise end up in landfills or waste incinerators.
Almost any organic material can be processed in this manner. This includes biodegradable waste materials such as waste paper, grass clippings, leftover food, sewage, and animal waste. Alternatively, anaerobic digesters can be fed with specially grown energy crops to boost biogas production. After sorting or screening to remove inorganic or hazardous materials such as metals and plastics, the material to be processed is often shredded or minced to achieve a better reaction (ultrasound has even been used in the process to aid in the break up of solids). Breaking the material into smaller pieces provides the bacteria with more surface area, allowing them to complete the process quicker. The material is then fed into a sealed digester. In the case of dry materials, water is added.
Anaerobic digestion generates biogas with a high proportion of methane that may be used to both heat the tank and run engines or microturbines for other on-site processes. In large treatment plants, sufficient energy can be generated in this way to produce more electricity than the machines require. The methane generation is a key advantage of the anaerobic process. Its key disadvantage is the long time required for the anaerobic process (up to 30 days) and the high capital cost.
The Goldbar Wastewater Treatment Plant in Edmonton, Alberta, Canada, currently uses the process (Fig. 7.2). Under laboratory conditions, it is possible to directly generate useful amounts of electricity from organic sludge using naturally occurring electrochemically active bacteria. Potentially, this technique could lead to an ecologically positive form of power generation, but in order to be effective such a microbial fuel cell must maximize the contact area between the effluent and the bacteria-coated anode surface, which could severely hamper throughput.
Vermistabilization is a technology that utilizes earthworms to stabilize and dewater wastewater sludge. It provides an all-in-one approach to sludge treatment. The technology works only for sludges with sufficient organic
matters and nutrients to support the earthworm population. The earthworm species Eisenia foetida has been shown to be the best worm species due to its growth rate and reproductive responses with temperatures ranging from 20-25°C (68-77°F). Although vermistabilization has been used and studied in composting in many nations, its application in sludge treatment is far from certainty. There are a number of critical issues that prevent vermista-bilization from being a common practice; among them is that earthworms can and do accumulate heavy metals and other organic pollutants. Past studies also found that some viruses, bacteria, or parasites could pass through the guts of earthworms and survive. In addition, certain industrial or even municipal sludges contain or do not contain certain substances that adversely affect the growth and reproduction of earthworms. These short comings of vermistabilization may not be crucial for many food-processing wastes; however, if food and agricultural wastewaters mix with other industrial or municipal wastewater or food and agricultural waste-waters contain toxic materials or heavy metals, vermistabilization may not be applicable to the sludge originated from these wastewaters.
Composting is also an aerobic process for the concurrent stabilization and dewatering of sludges. It involves mixing the wastewater solids with sources of carbon such as sawdust, straw, or wood chips to enable the biological process. In the presence of oxygen, bacteria digest both the sludge and the added carbon source and, in doing so, produce a large amount of heat. There are three basic types of compost systems (Reed et al., 1995): windrow, static pile, and enclosed reactors.
In a windrow system, the mixture of sludge and wood chips to be composted is placed in long rows, which are periodically turned and mixed to expose new surfaces to oxygen in the air.
Static pile systems consist of a porous base made from wood chips or compost in which the air is blown or drawn through either perforated or nonperforated pipe. The wood chips and sludge are piled on top of the porous base and screened compost covers the sludge-wood chip mixture.
Enclosed reactors look more or less like miniature compost containers used by home gardeners. Inside the reactors, there could be static pile or windrow-type layouts; the enclosure is usually for odor control.
Table 7.3 provides a general guideline for designing compost systems for sludge treatment. Monitoring process parameters is essential in any composting operation—it ensures efficient operation and quality of final products. Critical parameters such as moisture, oxygen concentration, heavy metals and organics, pathogens, pH, and temperature need to be watched closely and continuously for successful operations and compliance of laws and regulations.
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