Here is where we are with respect to the syllabus:
III. Waste management methods
A. Utilization
B. Biological Treatment
1. land application
2. in-vessel
a. aerobic
b. anaerobic
c. composting
Background for Topic
Biological treatment is decomposition of the organic residue into relatively stable, non-decomposable, materials. These materials are CO2, humus, and soluble organic and inorganic compounds. Application of the above concepts to mass and energy flows through a waste treatment system will be considered.
Biological Treatment Fundamentals
What do we mean, Biological Treatment? - We have seen in previous lectures that waste is a composite of organic and inorganic compounds and may exist as suspended, floating, settled, or dissolved solids. There are many ways to treat agricultural waste. They may be categorized broadly as physical, biological, and chemical.
Physical treatment, referred to as primary treatment, involves physical separation to the waste according to size or density. Screens and filters can remove particulate solids. Solids more dense than water can settle out of the liquid. This is typically a first step in many waste treatment systems. Suggest how such systems might be used in a swine or dairy operation.
Chemical treatment requires the addition of a chemical substance to promote a chemical reaction such as precipitation, oxidation, or volatilization. The usual goal of chemical treatment is to prepare the remaining material to be released back to the environment. Thus nutrient removal or immobilization and disinfecting are the general processes of chemical treatment, which is referred to as tertiary treatment. It is generally a final stage of the overall process. An example is the addition of ozone or chlorine to kill any residual microorganisms. This is often a final step in a waste treatment system.
Secondary treatment, or biological treatment, refers to the transformation of the waste through its consumption by living organisms. The waste material, in its fresh state, is an enticing meal for many organisms. These organisms eat the waste, utilize it, and, in turn, excrete waste themselves. Much of the carbon is oxidized to CO2. A portion of the waste is not consumed by anything. As the organisms die, they contribute their body tissues to the overall mass.
The figure to the left shows a general process diagram for a biological treatment system. Waste enters the control volume from the left. Conversion of the waste to CO2 is a major activity. Nutrients with volatile forms (N and S) can also be lost from the system. The refractory substances accumulate in the CV along with the minerals. Populations of organisms fluctuate with food quantities and environmental parameters. Depending on the system, losses by leaching are possible.
At this point some explanation is needed for this model biological treatment system. Discuss how the model applies to land application, a swine lagoon, or a compost pile.
Some examples of biological treatment include waste lagoon systems, compost operations, direct land application, oxidation ponds, activated sludge reactors, and methane production facilities.
Microorganisms are prevalent or pervasive in almost every environment. Since they, like all other animals, have nutritional requirements, there must be food for them to consume. In biological treatment systems, there are two general processes that occur: Energy-synthesis processes and Endogenous respiration processes.
Energy-synthesis Processes
Energy -containing waste is metabolized by the microorganisms. These organisms produce more cellular material and more organisms. This immobilizes nutrients, and energy is stored in the organic substances that are produced. The rates at which these processes occur are functions of the properties of the system. These system properties include:
suitability of the waste as a food for the organisms
presence and concentrations of growth-inhibiting substances or toxins
substrate medium temperature
substrate pH
total nutrient status (law of the minimum applies here)
Endogenous Respiration Processes
Living organisms feed on this material. As waste is consumed, the nutritional level can drop below the requirements of the population of organisms. The organisms are no longer capable of synthesis, since there is no energy and/or nutrients with which to synthesize. The organisms eventually die or go into some protected state. As the microorganisms die and decompose, nutrients and energy are returned to the system.
These processes are generally simultaneous. When waste is available, energy-synthesis processes dominate, and organism populations increase. When waste is limiting, the energy source for the organisms is from within (i.e. endogenous), and respiration processes dominate. Microbial populations decrease.
Microbial solids decrease during endogenous respiration, but there is always a portion of the solids that can not be broken down. These solids remain in the system and are referred to as inert or refractory solids. As a rule of thumb, 20-25% of the synthesized microbial solids will remain. Biological treatment systems are designed to operate within the range of these two general process classes. For example, if high-quality effluent is desired, the treatment will operate well into the endogenous respiration phase. If the material is to be land applied, treatment may be considered complete after the energy-synthesis reactions are complete. (Removal of the BOD)
Basic Biological Processes
Biological processes can be categorized by several other characteristics. Some important groupings include:
1. according to the presence or absence of dissolved oxygen
2. whether or not photosynthesis occurs
3. according to the mobility of the organism.
According to the presence or absence of dissolved oxygen
If the system has access to fresh air, organic matter is oxidized by organisms utilizing the molecular oxygen, O2. The chemical energy from the oxidation reaction is the energy source for growth and respiration of the organism. This is referred to as aerobic respiration, and organisms that undergo aerobic respiration are called aerobes. In liquid systems, the molecular oxygen is dissolved in the water. As the oxygen is used, it must be replaced. How it is replaced is a means of classifying aerobic treatment systems.
If the system does not have access to fresh air, the organic matter may be oxidized utilizing oxidizing agents other than O2. Example oxidizing agents include CO2, SO4=, and NO3-, and partially oxidized organic compounds. These processes are also referred to as fermentation. The chemical energy from such oxidation reactions is the energy source for growth and respiration of the organisms. The energy released is generally much less than that from aerobic reactions.
Some organisms can use O2 or other oxidizing agents, depending on levels of molecular oxygen in the system. These organisms are referred to as being facultative. Anaerobic organisms that can not tolerate the presence of molecular oxygen are called obligate anaerobes.
Waste treatment systems may be designed for either aerobic or anaerobic operation. an aerobic system may become anaerobic under the following circumstances:
when organic matter settles to the bottom of the treatment vessel
when the system becomes overloaded and oxygen levels are depleted
within growths of organic material (discrete anaerobic locations).
Whether or not photosynthesis occurs
If the biological treatment system contains green plants (algae, etc.) and light is present, photosynthesis occurs. Carbon dioxide will be consumed, and oxygen will be released as the plants grow. Systems which utilize photosynthesis do so for one or both of the following reasons: the production of biomass and to help maintain the supply oxygen for the aerobic system.
According to the mobility of the organism
Biological treatment systems are of many types, based on how the organisms grow. If they are mixed with the waste and move with the waste flow, the system is referred to as a suspended growth reactor. Agitation of the liquid keeps the organisms in suspension. Examples of suspended growth systems include activated sludge units, aerated lagoons, oxidation ditches, and well-mixed anaerobic digesters.
If the microorganisms grow on a supporting surface and the waste flows over or through the support, the system is referred to as an adherent growth reactor. Examples of support media include large stones, slag, corrugated plastic sheets, and rotating disks.
A system such as an oxidation pond or unaerated lagoon may not fit into either category completely.
Microorganisms important in biological treatment include bacteria, fungi, algae, protozoa, rotifers, crustacia, insects, and worms.