Aerobic Biological Treatment: the Operating Envelope, the Microbial Indicators and the Sludge Stages
Aerobic treatment is the default choice whenever the organic load is moderate and the site can tolerate a quick, essentially odour-free process. It relies on the biochemical action of aerobic bacteria and facultative anaerobic bacteria, degrading organic matter on the premise that free oxygen is supplied. Reaction rates are relatively fast, the required reaction time is short, and the process produces essentially no offensive odour and is fairly sanitary - which is why it suits wastewater with a BOD5 concentration below 600 mg/L.
The Envelope: Six Variables
Six factors decide whether the biology performs. Dissolved oxygen in the wastewater should sit between 0.3 and 2 mg/L, a band in which both aerobic bacteria and facultative bacteria can carry out aerobic respiration. For aerobic treatment the pH belongs between 6 and 9. Water temperature between 20 and 40 degrees C is most suitable. Microorganisms need six essential nutrient elements: carbon, nitrogen, an energy source, growth factors such as vitamins, inorganic salts including potassium, calcium, magnesium and iron, and water. Toxic substances - most heavy metals such as zinc, copper, lead and chromium - are toxic and unfavourable to microbial survival, though if the concentration is raised gradually the population can to some extent adapt to the new environment and improve treatment efficiency. Influent organic concentration is generally expressed as BOD5 between 100 and 600 mg/L.
Biodegradability of the wastewater is expressed by the BOD5/COD ratio. When BOD5/COD is above 0.5, biological treatment is clearly effective; when it is below 0.3, biological treatment is unsuitable. That single ratio is usually the first thing checked when a new industrial stream is proposed for a biological plant, because it decides whether any of the rest of the design is worth doing.
Two Families: Suspended and Attached Growth
Aerobic biochemical treatment includes the activated sludge process and the biofilm process. Activated sludge configurations run to plug-flow, completely mixed, step aeration, adsorptive-regenerative, extended aeration, deep-shaft aeration, pure-oxygen aeration, oxidation ditch and sequencing batch reactor (SBR) activated sludge processes. The biofilm family covers biofilters, biological rotating discs or bio-discs, biological contact oxidation and biological fluidised-bed processes.
The activated sludge system has five components: the aeration tank as the main reaction body where organic matter is degraded and microorganisms proliferate; the secondary settling tank, which separates sludge from water to guarantee effluent quality and concentrates sludge so enough can be returned; the return system, which maintains tank sludge concentration and whose ratio adjusts operating conditions; excess sludge, one of the routes for removing organic matter and the way stable operation is maintained; and the oxygen supply system providing dissolved oxygen to the microorganisms.
Three Stages in the Tank
Using the organic pollutants in the wastewater as a culture medium, activated sludge is cultivated continuously under conditions of dissolved oxygen, and its adsorption-flocculation and oxidation-decomposition actions then purify the wastewater. Three stages are involved. First, adsorption: pollutants in the sewage are adsorbed and attached by the flocs formed by the activated sludge microorganisms during contact. Second, oxidation: under aerobic conditions the microorganisms use part of the adsorbed and ingested organic matter as nutrients to synthesise cell material, while another part is catabolised and releases energy. Third, floc formation and coagulation-sedimentation: the biomass synthesised during oxidation flocculates into flocs and is separated from the water by gravity sedimentation.
Proliferation of the microorganisms is simply the inevitable result of those aeration-tank reactions and of organic matter degradation - and the visible result of proliferation is more activated sludge.
What the Sludge Is Made Of
Well-developed activated sludge appears as yellow-brown flocculent granular matter, also called biological flocs, with a solid matter content below 1%. That solid matter has four components: living cells (Ma), residues of microbial endogenous metabolism (Me), non-biodegradable organic matter carried in with the raw wastewater (Mi), and inorganic matter carried in with the raw wastewater and attached to the activated sludge (Mii).
Reading the Biology Under a Microscope
Biological indicators give faster warning than any chemical analysis, because protozoa are larger individuals and can be evaluated under a microscope within a short time - fast and simple, so that when the biology shifts in an unfavourable direction measures can be taken before the system deteriorates further. Excessive aeration easily causes sludge ageing and the sludge turns greyish white, at which point amoebae and rotifers appear in large numbers. When dissolved oxygen is insufficient, low-oxygen-tolerant organisms proliferate: among bacteria the white Beggiatoa sulphur bacteria dominate, and among protozoa genera such as Trichoda appear. When the organic concentration of the sewage is extremely low, metazoans such as rotifers dominate. Under shock loading or an influx of toxic substances, the number of Aspidisca drops sharply.
Biofilm: What Changes
In the biofilm process, the film first adsorbs organic matter in the attached water layer, aerobic bacteria in the aerobic layer decompose it, and it then enters the anaerobic layer for anaerobic decomposition, while the flowing water layer washes away the aged biofilm so new biofilm can grow. The cycle repeats and the sewage is purified.
The microbial ecology differs in useful ways. Species are diversified - autotrophic and heterotrophic bacteria, fast and slow proliferators, algae, protozoa and metazoa, with wide-ranging types, abundant genera and species and long food chains - because there is no intense agitation and sludge age is long. The biological food chain runs bacteria to algae to protozoa to metazoa, and its length is why the sludge yield of the biofilm process is one-quarter lower than that of the activated sludge process. Long generation times are possible, so nitrifying and denitrifying bacteria persist and the process has denitrification and nitrogen removal function. In process design the reactor is divided into multiple stages, forming different distributions of dominant microbial genera, which favours full exertion of the metabolic functions of the film.
In practice that translates into strong resistance to shock from water-quality fluctuations; diversified biological phases whose combined action degrades macromolecular and refractory substances; less excess sludge than activated sludge; tolerance of a higher organic load; and no sludge-bulking phenomenon, convenient operation and management, and low power consumption. The disadvantages are that the one-off investment in the packing and its support structure is relatively large, and the packing is prone to clogging.
The typical flowsheet runs wastewater through a primary sedimentation tank before the biofilm reactor and the effluent through a secondary sedimentation tank afterwards. The primary tank removes suspended solids in advance, preventing the biofilm reactor from being blocked by large matter; the secondary tank removes the biofilm sloughed off the packing. Because biofilm moisture content is lower than that of activated sludge and sludge settling velocity is greater, the secondary sedimentation tank can be smaller in volume. With attached growth there is no need to return seed sludge and generally no secondary-settling-tank sludge return in biofiltration, though to dilute the raw wastewater and keep scouring the media layer, high-rate filters and tower biofilters often use effluent recirculation.
What Causes Bulking
Where activated sludge does run into trouble, three groups of causes are worth separating. Raw water quality matters: readily biodegradable organic carbon together with low pH readily causes sludge bulking, toxic and harmful substances affect treatment performance, and industrial wastewater lacking nitrogen and phosphorus hinders the growth and reproduction of microorganisms. Process parameters matter too - organic loading and activated sludge concentration have to be selected together - and adding chlorine to the return sludge suppresses bulking caused by filamentous bacteria effectively. Environmental conditions, particularly air and water temperature, set the metabolic rate that everything else depends on.