3.3 What are the essential processing parameters?
The aerobic digestion process is influenced by various factors such as sludge characteristics, system design and desired outcomes. Some general considerations for factors such as oxygen requirements, pH level, temperature, mixing and solids retention time, all of which are important to control the process, will be explained in this topic.
Oxygen/Air Supply
The level of dissolved oxygen (DO) is the most critical parameter in the aerobic digestion process. Adequate oxygen concentrations allow the biological process to take place and prevent odours. DO concentrations typically range from 1 – 2 mg/L, but are acceptable up to 3 mg/L.
The rate of oxygen use by the microorganisms depends on the rate of biological oxidation. The oxygen uptake rate (OUR) is used to determine the level of biological activity and the resulting solids destruction occurring in the digester. The OUR is expressed in milligrams of oxygen per gram of volatile suspended solids (VSS) per hour. The following graph shows the OUR in relation to solids retention time and temperature. Higher temperatures generally result in increased metabolic rates for microorganisms. This means that microorganisms become more active, leading to higher and quicker rates of oxygen consumption during biological oxidation.
Over-aeration should be avoided as it wastes energy and may have negative effects on the process performance. In the aeration process, microorganisms excrete a sticky film around their cells as their “food” gets depleted. Aeration keeps them in suspension, allowing them to collide and ultimately stick together, forming flocs. These flocs will settle and form a solid sludge. Over-aeration will break flocs apart, causing small and dispersed flocs that will not settle.
The dissolved oxygen level can also influence denitrification and nitrification. If nitrification is intended, then the DO range should be above 2 mg/L and the oxygen-reduction potential (ORP), which is a measure of the tendency of the liquid to either gain or lose electrons and an indication of an either oxidizing or reducing environment, should be +100 mv to +350 mv. On the other hand, denitrification occurs under anoxic conditions. By decreasing the DO, nitrate is further reduced to nitrogen gas. For denitrification, the DO range should be below 0.2 mg/L and the ORP should range between +50 mv to -50 mv.
An article about the critical role of aeration systems can be found under the materials tab.
Mixing
Mixing is essential as it ensures adequate contact between organisms and their food supply and ensures uniform distribution of oxygen throughout the digester. Mixing is commonly realized by the aeration system. It is recommended that the diffusers supply air at a rate of 0.3 to 0.6 L/m3s. In case the rate of aeration to meet the oxygen uptake rate is less than the rate needed to keep organisms in suspension, supplemental mixing (i.e., mechanical mixers) is required.
pH
The typical pH range for aerobic digestion is neutral to slightly alkaline, around pH 6.5 to 8.5. Maintaining the pH within this range supports the growth and metabolic activity of the microorganisms.
Two products of aerobic digestion that tend to lower the digester pH are carbon dioxide and hydrogen ions. A pH drop can occur when ammonia is oxidized to nitrate if the alkalinity of the wastewater is insufficient to buffer the solution. When the buffering capacity of the sludge is insufficient, it may be necessary to chemically adjust the pH. The proper chemical dosage can be determined by performing a bench-scale jar test and proportioning the chemical dosage from the jar test to the digester volume.
Temperature
The liquid temperature significantly affects the rate of volatile solids (VS) reduction in the aerobic digester. As with all biological processes, the higher the temperature, the greater the efficiency. At temperatures lower than 10°C the process is less effective. A mesophilic temperature range, between 20°C and 40°C, is desirable.
Solids Retention Time (SRT)
The SRT refers to the average amount of time that particulate matter or sludge solids spend in the aerobic digester. A sufficient SRT is necessary for microorganisms to complete their digestion of the sludge. It is defined as the total mass of biological solids in the reactor divided by the mass of solids removed from the process on a daily average:
Typically, increased SRT results in an increase in the degree of solids reduction. The exact correlation depends on the characteristics of the waste sludge being digested. The overall SRT typically ranges from 10 to 40 days. A high retention time will result in reduced dewaterability. Therefore, if dewaterability is important, SRT should be kept on the low end of the range.
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