Lagoon Systems - 4...

Lagoon Microbiology...

Microorganisms found in aerated and facultative ponds are more diverse than those observed in many other biological treatment processes, due to the diverse growth environments present. Both aerobic and anaerobic bacteria are involved as well as algae and some higher life forms such as protozoans, rotifers, daphnia, and insect larvae.

Aerobic Bacteria...

The aerobic bacteria that occur are similar to those found in other treatment processes such as activated sludge. Three functional groups occur : freely dispersed, single bacteria; floc-forming bacteria; and filamentous bacteria. All function similarly to oxidize organic carbon (BOD) to produce CO ₂ and new bacteria (new sludge).

Many bacterial species that degrade wastes grow as single bacteria dispersed in the wastewater. Although these readily oxidize BOD, they do not settle and hence often leave the lagoon system in the effluent as solids (TSS). These tend to grow in lagoons at high organic loading and low oxygen conditions. More important are the floc-forming bacteria, those that grow in a large aggregate (floc) due to exocellular polymer production (the glycocalyx). This growth form is important as these flocs degrade BOD and settle at the end of the process, producing a low TSS effluent.

A number of fiIlamentous bacteria occur in lagoons, usually at specific growth environments. These generally do not cause any operational problems in lagoons, in contrast to activated sludge where filamentous bulking and poor sludge settling is a common problem.

Most heterotrophic bacteria have a wide range in environmental tolerance and can function effectively in BOD removal over a wide range in pH and temperature. Aerobic BOD removal generally proceeds well from pH 6.5 to 9.0 and at temperatures from 3 - 4 ^O C to 60 - 70 ^O C (mesophilic bacteria are replaced by thermophilic bacteria at temperatures above 35 ^O C). BOD removal generally declines rapidly below 3 - 4 ^O C and ceases at 1 - 2 ^O C. A very specialized group of bacteria occurs to some extent in lagoons (and other wastewater treatment systems) that can oxidize ammonia via nitrite to nitrate, termed nitrifying bacteria. These bacteria are strict aerobes and require a redox potential of at least + 200 m V (Holt et al., 1994). It was once thought that only two bacteria were involved in nitrification : Nitrosomonas europaea, which oxidizes ammonia to nitrite, and Nitrobacter winogradskyi, which oxidizes nitrite to nitrate. It is now known that at least 5 genera of bacteria oxidize ammonia and at least three genera of bacteria oxidize nitrite (Holt et al., 1994). Besides oxygen, these nitrifying bacteria require a neutral pH ( 7 - 8 ) and substantial alkalinity ( these autotrophs use CO ₂ as a carbon source for growth). This indicates that complete nitrification would be expected at pond pH values between pH 7.0 and 8.5. Nitrification ceases at pH values above pH 9 and declines markedly at pH values below 7. This results from the growth inhibition of the nitrifying bacteria. Nitrification, however, is not a major pathway for nitrogen removal in lagoons. Nitrifying bacteria exists in low numbers in lagoons. They prefer attached growth systems and / or high MLSS sludge systems.

Anaerobic Bacteria...

Anaerobic, heterotrophic bacteria that commonly occur in lagoons are involved in methane formation (acid-fonning and methane bacteria) and in sulfate reduction (sulfate reducing bacteria). Anaerobic methane formation involves three different groups of anaerobic bacteria that function together to convert organic materials to methane via a three step process. General anaerobic degraders - many genera of anaerobic bacteria hydrolyze proteins, fats, and poly saccharides present in wastewater to amino acids, short-chain peptides, fatty acids, glycerol, and mono- and di-saccharides. These have a wide environmental tolerance in pH and temperature. Acid-forming bacteria - this diverse group of bacteria converts products from above under anaerobic conditions to simple alcohols and organic acids such as acetic, propionic, and butyric. These bacteria are hardy and occur over a wide pH and temperature range. Methane forming bacteria - these bacteria convert formic acid, methanol, methylamine, and acetic acid under anaerobic conditions to methane. Methane is derived in part from these compounds and in part from CO ₂ reduction. Methane bacteria are environmentally sensitive and have a narrow pH range of 6.5 - 7.5 and require temperatures > 14 ^O C.

Note that the products of the acid formers (principally acetic acid) become the substrate for the methane producers. A problem at times exists where the acid formers overproduce organic acids, lowering the pH below where the methane bacteria can function ( a pH < 6.5 ). This can stop methane fonnation and lead to a buildup of sludge in a lagoon with a low pH. In an anaerobic fennenter, this is called a "stuck digester". Also, methane fennentation ceases at cold temperature, probably not occurring in most lagoons in the wintertime in cold climates. A number of anaerobic bacteria (14 genera reported to date (Bolt et al., 1994)) called sulfate reducing bacteria can use sulfate as an electron acceptor, reducing sulfate to hydrogen sulfide. This occurs when BOD and sulfate are present and oxygen is absent. Sulfate reduction is a major cause of odors in ponds.

Photosynthetic Organisms...

Anaerobic, photosynthetic bacteria occur in all lagoons and are the predominant photosynthetic organisms in anaerobic lagoons, The anaerobic sulfur bacteria, generally grouped into the red and green sulfur bacteria and represented by about 28 genera (Ehrlich, 1990), oxidize reduced sulfur compounds ( H ₂ S ) using light energy to produce sulfur and sulfate, Here, H ₂ S is used in place of H ₂ O as used by algae and green plants, producing S0 ₄ ^- instead of O ₂ .

All are either strict anaerobes or microaerophilic. Most common are Chromatium, Thiocystis, and Thiopedia, which can grow in profusion and give a lagoon a pink or red color. Finding them is most often an indication of organic overloading and anaerobic conditions in an intended aerobic system. Conversion of odorous sulfides to sulfur and sulfate by these sulfur bacteria is a significant odor control mechanism in facultative and anaerobic lagoons, and can be desirable. Algae are aerobic organisms that are photosynthetic and grow with simple inorganic compounds ( CO ₂ , NH ₃ , NO ₃ ^- , and PO ₄ ^{- 2} ) using light as an energy source. Note that algae produce oxygen during the daylight hours and consume oxygen at night.

Algae are desirable in lagoons as they generate oxygen needed by bacteria for waste stabilization. Three major groups occur in lagoons, based on their chlorophyll type: brown algae ( diatoms ), green algae, and red algae. The predominant algal species at any given time is dependent on growth conditions, particularly temperature, organic loading, oxygen status, nutrient availability , and predation pressures. A fourth type of "algae" common in lagoons is the cyano-bacteria or blue-green bacteria. These organisms grow much as the true algae, with the exception that most species can fix atmospheric nitrogen. Blue-green bacteria often bloom in lagoons and some species produce odorous and toxic by-products. Blue-green bacteria appear to be favored by poor growth conditions including high temperature, low light, low nutrient availability ( many fix nitrogen ) and high predation pressure. Common blue-green bacteria in waste treatment systems include Aphanothece, Microcystis, Oscillatoria and Anabaena. Algae can bloom in lagoons at any time of the year ( even under the ice) ; however, a succession of algal types occurs over the season.

There is also a shift in the algal species present in a lagoon through the season, caused by temperature and rotiter and Daphnia predation. Diatoms usually predominate in the wintertime at temperatures < 60 ^O F. In the early spring when predation is low and lagoon temperatures increase above 60 ^O F, green algae such as Chlorella, Chlamydomonas, and Euglena often predominate in waste treatment lagoons. The predominant green algae change to species with spikes or horns such as Scenesdesmus, Micractinium, and Ankistrodesmus later in the season when Rotifers and Daphnia are active ( these species survive predation better ).

Algae grow at warmer temperature, longer detention time, and when inorganic minerals needed for growth are in excess. Alkalinity ( inorganic carbon ) is the only nutrient likely to be limiting for algal growth in lagoons. Substantial sludge accumulation in a lagoon may become soluble upon warming in the spring, releasing algal growth nutrients and causing an algal bloom. Sludge resolution of nutrients is a major cause of high algal growth in a lagoon, requiring sludge removal from the lagoon for correction.

The pH at a treatment lagoon is determined by the various chemical species of alkalinity that are present. The main species present are carbon dioxide ( CO ₂ , bicarbonate ion HCO ₃ ^- , and carbonate ion CO ₃ ^{- 2} ). High amounts of CO ₂ yield a low lagoon pH, while high amounts of CO ₃ ^{- 2} yield a high lagoon pH. Bacterial growth on BOD releases CO ₂ which subsequently dissolves in water to yield carbonic acid ( H ₂ CO ₃ ). This rapidly dissociates to bicarbonate ion, increasing the lagoon alkalinity. Bacterial oxidation of BOD causes a decrease in lagoon pH due to CO ₂ release.

Algal growth in lagoons has the opposite effect on lagoon pH, raising the pH due to algal use for growth of inorganic carbon ( CO ₂ and HCO ₃ ^- ). Algal growth reduces the lagoon alkalinity which may cause the pH to increase if the lagoon alkalinity ( pH buffer capacity ) is low. Algae can grow to such an extent in lagoons ( a bloom ) that they consume for photosynthesis all of the CO ₂ and HCO ₃ ^- present, leaving only carbonate ( CO ₃ ^{- 2} ) as the pH buffering species. This causes the pH of the lagoon to become alkaline. pH values of 9.5 or greater are common in lagoons during algal blooms, which can lead to lagoon effluent pH violations ( in most states this is pH = 9 ). It should be noted that an increase in the lagoon pH caused by algal growth can be beneficial. Natural disinfection of pathogens is enhanced at higher pH. Phosphorus removal by natural chemical precipitation is greatly enhanced at pH values greater than pH = 8.5. In addition, ammonia stripping to the atmosphere is enhanced at higher pH values ( NH ₃ is strippable, not NH ₄ ⁺ ).

Protozoans and Microinvertebrates...

Many higher life forms ( animals ) develop in lagoons. These include protozoans and microinvertebrates such as rotifers, daphnia, annelids, chironomids ( midge larvae ), and mosquito larvae ( often termed the zooplankton). These organisms playa role in waste purification by feeding on bacteria and algae and promoting flocculation and settling of particulate material. Protozoans are the most common higher life forms in lagoons with about 250 species identified in lagoons to date (Curds, 1992). Rotifers and daphnia are particularly important in controlling algal overgrowth and these often "bloom" when algal concentrations are high. These microinvertebrates are relatively slow growing and generally only occur in systems with a detention time of > 10 days. Mosquitos grow in lagoons where shoreline vegetation is not removed and these may cause a nuisance and public health problem. Culex tarsalis, the vector of Western Equine Encephalitis in the western U.S., grows well in wastewater lagoons (USEPA, 1983). The requirement for a minimum lagoon bank slope and removal of shoreline vegetation by most regulatory agencies is based on the public health need to reduce mosquito vectors.

"Lagoon Microorganisms"...