Anaerobic waste water treatment methods. Bioreactors for
bio-methanation and bio-filters

Joint anaerobic municipal wastewater and sludge treatment

1. UASB-technology

The UASB-process (upflow anaerobic sludge blanket) has proven to be the most promising communal or municipal anaerobic low-cost treatment technology. It can now be considered feasible for municipal wastewater treatment because of its proven rapid organic removal efficiency, its simplicity and low degree of mechanization, the low capital and maintenance costs and low land and energy requirements.

A UASB-reactor is constructed with a specific feeding system consisting of inlet pipes equally delivering influent to the bottom of the unit. The upstream velocity is in equilibrium with the sludge settling speed, so that a suspended sludge (bacterial) blanked is formed. The upstream velocity has to be rather constant in order to guarantee a proper sludge-water contact and to avoid a washout of the active bacteria; an external mixing device for this technology is not required.

Besides the distribution system, the most characteristic device is the “gas-liquid-solid” or “three-phase separator” at the top of the reactor. Its function is to separate the biogas and to retain the solids (bacterial sludge) and the treated liquid phase, thus preventing sludge washout. Due to their anaerobic operation, UASB-reactors are characterized by a considerably lower sludge production (the most relevant cost factor in municipal wastewater treatment) and a low energy demand, thus leaving a net energy surplus.

The pathogen removal efficiency of UASB treatment processes is not considered sufficient if environmental standards from industrial countries are applied, in particular not for the sludge, and must be followed by a post-treatment option to meet the increasingly strict discharge standards. Nevertheless, already now a 90-99% removal of for example helminth eggs in the effluent wastewater is possible with UASB technology alone. Further treatment options may include composting of digested sludge for final pathogen reduction in the sludge and wastewater post-treatment in ponds or constructed wetland systems

2 Septic Tank

The septic tank is an appropriate low cost technology and the most common, small scale, decentralised anaerobic treatment plant, however built without any gas collection or utilization system. It is a simple sedimentation tank with a low requirement for maintenance and a treatment capacity of up to about 50 households. The system consists of a closed tank where sedimentation takes place and settleable solids are retained. Retention time of the liquid is in the order of one day. Sludge is digested anaerobically in the septic tank, resulting in a reduced volume of sludge. Based on the low removal efficiencies of 30% COD, 50% BOD and 70% TSS respectively and low nutrient removal, the effluent is destined for use in agricultural irrigation.

3. Anaerobic sludge treatment in activated sludge processes (Sewage sludge digester)

 Anaerobic treatment of sludge from aerobic wastewater treatment with long retention times has a very long history in some of the central European countries but has improved considerably: These anaerobic systems can be built and operated on various scales in size with a high degree of technical sophistication and automation, but sometimes are technically quite simple as well. Anaerobic sewage sludge treatment offers several substantial advantages: ¾ Reduction of sludge volumes, Stabilisation of the sludge, Production of biogas to be used as process  energy, retention of Valuable nutrients. Anaerobic sludge can be preserved and easier dewatered.

Sewage sludge is the total solid material that results from sedimentation and bacterial activity and growth during aerobic wastewater treatment. The floating and sinking layers formed before, during and after a treatment of the wastewater are normally all fed to the sludge digester. Here, anaerobic fermentation takes place at process temperatures of 35°C (mesophilic) to 55°C (thermophilic) and biogas is generated. To generate appropriate reactor temperatures, a heating system is required. Its energy demand can partly, sometimes fully, be covered by utilising the produced gas, which can either be burnt directly or in cogeneration units.

Bioreactor

Configuration

Anaerobic digesters can be designed and engineered to operate using a number of different configurations and can be categorized into batch vs. continuous process mode, mesophilic vs. thermophilic temperature conditions, high vs. low portion of solids, and single stage vs. multistage processes. More initial build money and a larger volume of the batch digester is needed to handle the same amount of waste as a continuous process digester. Higher heat energy is demanded in a thermophilic system compared to a mesophilic system and has a larger gas output capacity and higher methane gas content. For solids content, low will handle up to 15% solid content. Above this level is considered high solids content and can also be known as dry digestion. In a single stage process, one reactor houses the four anaerobic digestion steps. A multistage process utilizes two or more reactors for digestion to separate the methanogenesis and hydrolysis phases.

Batch or continuous

Anaerobic digestion can be performed as a batch process or a continuous process. In a batch system, biomass is added to the reactor at the start of the process. The reactor is then sealed for the duration of the process. In its simplest form batch processing needs inoculation with already processed material to start the anaerobic digestion. In a typical scenario, biogas production will be formed with a normal distribution pattern over time. Operators can use this fact to determine when they believe the process of digestion of the organic matter has completed. There can be severe odour issues if a batch reactor is opened and emptied before the process is well completed. A more advanced type of batch approach has limited the odour issues by integrating anaerobic digestion with in-vessel composting. In this approach inoculation takes place through the use of recirculated degasified percolate. After anaerobic digestion has completed, the biomass is kept in the reactor which is then used for in-vessel composting before it is opened^[28] As the batch digestion is simple and requires less equipment and lower levels of design work, it is typically a cheaper form of digestion.^[29] Using more than one batch reactor at a plant can ensure constant production of biogas.

In continuous digestion processes, organic matter is constantly added (continuous complete mixed) or added in stages to the reactor (continuous plug flow; first in – first out). Here, the end products are constantly or periodically removed, resulting in constant production of biogas. A single or multiple digesters in sequence may be used. Examples of this form of anaerobic digestion include continuous stirred-tank reactors, upflow anaerobic sludge blankets, expanded granular sludge beds, and internal circulation reactors.

High solids (dry) digesters are designed to process materials with a solids content between 25 and 40%. Unlike wet digesters that process pumpable slurries, high solids (dry – stackable substrate) digesters are designed to process solid substrates without the addition of water. The primary styles of dry digesters are continuous vertical plug flow and batch tunnel horizontal digesters. Continuous vertical plug flow digesters are upright, cylindrical tanks where feedstock is continuously fed into the top of the digester, and flows downward by gravity during digestion. In batch tunnel digesters, the feedstock is deposited in tunnel-like chambers with a gas-tight door. Neither approach has mixing inside the digester. The amount of pretreatment, such as contaminant removal, depends both upon the nature of the waste streams being processed and the desired quality of the digestate. Size reduction (grinding) is beneficial in continuous vertical systems, as it accelerates digestion, while batch systems avoid grinding and instead require structure (e.g. yard waste) to reduce compaction of the stacked pile. Continuous vertical dry digesters have a smaller footprint due to the shorter effective retention time and vertical design. Wet digesters can be designed to operate in either a high-solids content, with a total suspended solids (TSS) concentration greater than ~20%, or a low-solids concentration less than ~15%.

High solids (wet) digesters process a thick slurry that requires more energy input to move and process the feedstock. The thickness of the material may also lead to associated problems with abrasion. High solids digesters will typically have a lower land requirement due to the lower volumes associated with the moisture. High solids digesters also require correction of conventional performance calculations (e.g. gas production, retention time, kinetics, etc.) originally based on very dilute sewage digestion concepts, since larger fractions of the feedstock mass are potentially convertible to biogas.

Low solids (wet) digesters can transport material through the system using standard pumps that require significantly lower energy input. Low solids digesters require a larger amount of land than high solids due to the increased volumes associated with the increased liquid-to-feedstock ratio of the digesters. There are benefits associated with operation in a liquid environment, as it enables more thorough circulation of materials and contact between the bacteria and their food. This enables the bacteria to more readily access the substances on which they are feeding, and increases the rate of gas production.

Complexity

Digestion systems can be configured with different levels of complexity. In a single-stage digestion system (one-stage), all of the biological reactions occur within a single, sealed reactor or holding tank. Using a single stage reduces construction costs, but results in less control of the reactions occurring within the system. Acidogenic bacteria, through the production of acids, reduce the pH of the tank. Methanogenic bacteria, as outlined earlier, operate in a strictly defined pH range. Therefore, the biological reactions of the different species in a single-stage reactor can be in direct competition with each other. Another one-stage reaction system is an anaerobic lagoon. These lagoons are pond-like, earthen basins used for the treatment and long-term storage of manures. Here the anaerobic reactions are contained within the natural anaerobic sludge contained in the pool.

In a two-stage digestion system (multistage), different digestion vessels are optimised to bring maximum control over the bacterial communities living within the digesters. Acidogenic bacteria produce organic acids and more quickly grow a=nd reproduce than methanogenic bacteria. Methanogenic bacteria require stable pH and temperature to optimise their performance.

The residence time in a digester varies with the amount and type of feed material, and with the configuration of the digestion system. In a typical two-stage mesophilic digestion, residence time varies between 15 and 40 days, while for a single-stage thermophilic digestion, residence times is normally faster and takes around 14 days. The plug-flow nature of some of these systems will mean the full degradation of the material may not have been realised in this timescale. In this event, digestate exiting the system will be darker in colour and will typically have more odour.

In the case of an upflow anaerobic sludge blanket digestion (UASB), hydraulic residence times can be as short as 1 hour to 1 day, and solid retention times can be up to 90 days. In this manner, a UASB system is able to separate solids and hydraulic retention times with the use of a sludge blanket. Continuous digesters have mechanical or hydraulic devices, depending on the level of solids in the material, to mix the contents, enabling the bacteria and the food to be in contact. They also allow excess material to be continuously extracted to maintain a reasonably constant volume within the digestion tanks

Biofilter

Biofiltration was first introduced in England in 1893 as a trickling filter for wastewater treatment and has since been successfully used for the treatment of different types of water. Biological treatment has been used in Europe to filter surface water for drinking purposes since the early 1900s and is now receiving more interest worldwide. Biofiltration is also common in wastewater treatment, aquaculture and grey water recycling, as a way to minimize water replacement while increasing water quality.

Biofiltration process

A biofilter is a bed of media on which microorganisms attach and grow to form a biological layer called biofilm. Biofiltration is thus usually referred to as a fixed–film process. Generally, the biofilm is formed by a community of different microorganisms bacteria, fungi, yeast, etc.), macro-organisms (protozoa, worms, insect’s larvae, etc.) and extracellular polymeric substances (EPS). The aspect of the biofilm is usually slimy and muddy.

Water to be treated can be applied intermittently or continuously over the media, via upflow or downflow. Typically, a biofilter has two or three phases, depending on the feeding strategy (percolating or submerged biofilter):

·         a solid phase (media);

·         a liquid phase (water);

·         a gaseous phase (air).

Organic matter and other water components diffuse into the biofilm where the treatment occurs, mostly by biodegradation. Biofiltration processes are usually aerobic, which means that microorganisms require oxygen for their metabolism. Oxygen can be supplied to the biofilm, either concurrently or countercurrently with water flow. Aeration occurs passively by the natural flow of air through the process (three phase biofilter) or by forced air supplied by blowers.

Microorganisms' activity is a key-factor of the process performance. The main influencing factors are the water composition, the biofilter hydraulic loading, the type of media, the feeding strategy (percolation or submerged media), the age of the biofilm, temperature, aeration, etc.

Types of filtering media

Originally, biofilter was developed using rock or slag as filter media, but different types of material are used today. These materials are categorized as inorganic media (sand, gravel, geotextile, different shapes of plastic media, glass beads, etc.) and organic media (peat, wood chips, coconut shell fragments, compost, etc.)

Advantages

Although biological filters have simple superficial structures, their internal hydrodynamics and the microorganisms' biology and ecology are complex and variable.^[6] These characteristics confer robustness to the process. In other words, the process has the capacity to maintain its performance or rapidly return to initial levels following a period of no flow, of intense use, toxic shocks, media backwash (high rate biofiltration processes), etc.

The structure of the biofilm protects microorganisms from difficult environmental conditions and retains the biomass inside the process, even when conditions are not optimal for its growth. Biofiltration processes offer the following advantages:

·         Because microorganisms are retained within the biofilm, biofiltration allows the development of microorganisms with relatively low specific growth rates;

·         Biofilters are less subject to variable or intermittent loading and to hydraulic shock;^[7]

·         Operational costs are usually lower than for activated sludge;

·         Final treatment result is less influenced by biomass separation since the biomass concentration at the effluent is much lower than for suspended biomass processes;

·         Attached biomass becomes more specialized (higher concentration of relevant organisms) at a given point in the process train because there is no biomass return.

Drawbacks

Because filtration and growth of biomass leads to an accumulation of matter in the filtering media, this type of fixed-film process is subject to bioclogging and flow channeling. Depending on the type of application and on the media used for microbial growth, bioclogging can be controlled using physical and/or chemical methods. Whenever possible, backwash steps can be implemented using air and/or water to disrupt the biomat and recover flow. Chemicals such as oxidizing (peroxide, ozone) or biocide agents can also be used.