Algae are primarily aquatic, autotrophic, photosynthetic organisms that range from being unicellular to living as large aggregates of filamentous cells. Also referred to as phytoplankton, algae play the role of primary producers in aquatic environments. They all contain chlorophyll a; some contain chlorophyll b and c as well as other pigments such as xanthophyll and carotenoids, thus enabling them to carry out oxygenic photosynthesis.
Currently algae are not extensively utilised within the activated sludge process however newer process designs incorporate algal ponds as a side stream process to the conventional activated sludge process. Algae are more predominantly involved with biological transformations in facultative or maturation ponds. Within these ponds algae are involved in nutrient uptake of mainly nitrogen and phosphorus. Some algae are able to fix nitrogen, while most others utilize ammonium or nitrate. Algae are also able to concentrate silica, and uptake a wide range of heavy metals from solution, thus allowing for the removal of heavy metals that the conventional BNR process does not account for. Additionally, photosynthesis leads to an increase in pH, which may create good conditions for removal of nutrients by other means. Most importantly, algal photosynthesis produces oxygen, which is used by heterotrophic microorganisms for further biochemical transformations.
Generally only one or two species will be dominant at any one time in a facultative pond. The most commonly recorded genera are: Chlorella, Scenedesmus, Chlamydomonas, Micractinium, Euglena, Ankistrodesmus, Oscillatoria, and Microcystis. Motile genera such as Chlamydomonas, Pyrobotrys, and Euglena can alter their vertical position in the water column to take maximum advantage of light intensity and temperature, unlike non-motile genera such as Chlorella. The dominant algal species is determined by the organic loading with those algae able to tolerate anaerobic conditions being recorded in ponds receiving heavy organic loads.
Changes in the colour of the pond are also a useful guide to changes in condition. Healthy ponds are green or brownish due to the presence of green algae. However, a change to blue-green indicates a change in algal dominance from green to blue-green algae and as the latter are less efficient than the green algae, indicating a deterioration in conditions. Changes in colour to pink, red or reddish-brown in the summer or autumn are caused by blooms of photosynthetic sulphur bacteria because of an increase in sulphate or sulphide concentration resulting from increased anaerobiosis.
Often referred to as blue-green algae, cyanobacteria are prokaryotic organisms that differ from photosynthetic bacteria in the fact that they carry out oxygenic photosynthesis. They contain chlorophyll a and accessory pigments such as phycocyanin (blue pigment) and phycoerythrin (red pigment). The characteristic bluegreen color exhibited by these organisms is due to the combination of chlorophyll a and phycocyanin. Cyanobacteria occur as unicellular, colonial, or filamentous organisms. Cyanobacteria are ubiquitous and, owing to their resistance to extreme environmental conditions (e.g., high temperatures, desiccation), they are found in desert soils and hot springs. They are responsible for algal blooms in lakes and other aquatic environments, and some are quite toxic.
Protozoa and Metazoa are mostly complex, highly organised, single celled organisms. They are mostly aerobic heterotrophs, classified by life cycle and means of locomotion.
Protozoa are common components in activated sludge with population densities reaching up to 50,000 organisms/ml, representing as much as 5–12% of the dry weight of the mixed liquor. Over 228 species of Protozoa haver been isolated from activated sludge plants with 70% of them from the class Ciliatea. Flagellates, both Phyto- and Zoomastigophorea, are far more common in percolating filters than in the activated sludge process and are commonly an indication of overloaded plants.
Protozoa are important both as disease causing organisms and as a vital link in the food chain from bacteria on up. They feed on bacteria and in turn are food for larger organisms and act as polishers of effluent from waste treatment processes by consuming bacteria and particulate matter.Changes in the protozoan community reflect those of the plant operating conditions, namely F/M ratio, nitrification, sludge age, or dissolved oxygen level in the aeration tank. The protozoan species composition of activated sludge may indicate the BOD removal efficiency of the process. For example, the presence of large numbers of stalked ciliates and rotifers indicate a low BOD.
Rotifers are the most abundant macro invertebrates found in the activated sludge process. Rotifers move by swimming freely or crawling. They are very small and soft except for their jaws. Rotifers are often in the same size range as ciliates and have a contractile foot that allows them to attach to activated sludge floc. They are beneficial in stabilizing organic wastes, stimulating microfloral activity and decomposition, enhancing oxygen penetration, and recycling mineral nutrients. The presence of rotifers in activated sludge generally means a good, stable sludge with plenty of oxygen.
The principal role of rotifers in wastewater is the removal of bacteria and the development of floc. Rotifers contribute to the removal of effluent turbidity by removing non-flocculated bacteria. Mucous secreted by rotifers at either the mouth opening or from the foot aids in better floc formation. Rotifers usually are present in older sludge becasue they require a longer time to become established in the treatment process and indicate increasing stabilization of organic wastes and lower BOD and TSS (Total Suspended Solids). Rotifers are rarely found in large number in wastewater treatment processes.
Although bacteria are primarily responsible for the removal of organic matter, protozoans also play an important role in the purification process as bacterial feeders. This has three major effects:
1. Regulation of bacterial density: prevents bacteria from reaching self-limiting numbers, causing enhanced bacterial activity as well as reducing the overall bacterial biomass.
2. They clarify effluent by removing suspended material. In nutrient limited activated sludge, protozoal grazing will reduce the bacterial population, resulting in an increase in substrate concentration in the effluent or enhanced nutrient uptake rates by the remaining bacteria.
3. Under certain food-limiting conditions, the dispersed bacteria well be in competition with the floc-forming bacteria for the soluble substrate present. By removing the dispersed bacteria, the protozoans will reduce the competition for the remaining substrate, allowing the floc-forming bacteria to produce larger flocs, which will improve settleability of the mixed liquor as well as reducing suspended material in the effluent, thereby enhancing overall quality.
They are also capable of ingesting Cryptosporidium oocysts in wastewater and can thus serve as vectors for the transmission of this parasite.
Fungi are commonly associated with all wastewater units, including sludge tanks and anaerobic digesters, although they are most prevalent in the film of percolating filters. Large amounts of fungi in the filter beds are considered to be undesirable as they can cause blockage, which impedes drainage and aeration and eventually lead to ponding.
Fungal filaments are rarely dominant within activated sludge flocs, although 4 common genera are always present, with Geotrichium candidum and Trichosporon species being the most abundant. Heterotrophic fungi can be as efficient as heterotrophic bacteria in the removal of organic matter from wastewater however fungi produce a greater biomass per unit weight of substrate utilised than bacteria in wastewater treatment units, resulting in more sludge production per kg BOD removed, thus causing adverse sludge effects.
Fungi grow optimally in low pH which inhibits bacterial growth, thus this can be used as an indicator for the presence of acidic industrial effluent where it will outgrow the inhibited bacterial population. When fungi dominate, the reduced density of flocs reduces their settling velocity and may eventually lead to bulking. Additionally the predacious fungus, Zoophagous insidians captures and consumes rotifers. And can thus play a role in both floc formation and in the regulation of the nematode population density.
Although fungi are also present in ponds they are not important and are thought to be restricted due to the high pH caused by the photosynthetic activity of the algae.
Viruses are ultramicroscopic, parasitic organisms that propagate through the infestation of a host organism. Viruses excreted with human and animal faeces are called enteric viruses.
The viral diversity within the activated sludge system may include the:
• hepatitis A virus;
• the adenoviruses, and
Many of these viruses, especially the enteroviruses, are hardy and survive for long periods in the environment and thus pose serious health hazards if allowed to contaminate potable water supplies and food.
Temperature and solar radiation are important factors that control virus persistence in oxidation ponds. Virus removal is expected to be high under a hot and sunny climate in the upper layer of the pond. Other biological factors involved in virus inactivation in ponds includes: high pH resulting from heavy growth of algae, and adsorption to suspended solids that settle in pond sediments.