Secondary treatment
[1]: 11 The aim is to achieve a certain degree of effluent quality in a sewage treatment plant suitable for the intended disposal or reuse option.
During secondary treatment, biological processes are used to remove dissolved and suspended organic matter measured as biochemical oxygen demand (BOD).
Fixed-film or attached growth systems include trickling filters, constructed wetlands, bio-towers, and rotating biological contactors, where the biomass grows on media and the sewage passes over its surface.
With this aerobic oxidation and nitrification, the organic solids are converted into biofilm grazed by insect larvae, snails, and worms which help maintain an optimal thickness.
Overloading of beds may increase biofilm thickness leading to anaerobic conditions and possible bioclogging of the filter media and ponding on the surface.
Activated sludge plants encompass a variety of mechanisms and processes using dissolved oxygen to promote growth of biological floc that substantially removes organic material.
Overflow from the activated sludge mixing chamber is sent to a clarifier where the suspended biological floc settles out while the treated water moves into tertiary treatment or disinfection.
Nocardia, a floating brown foam sometimes misidentified as sewage fungus, is the best known of many different fungi and protists that can overpopulate the floc and cause process upsets.
[18] One type of system that combines secondary treatment and settlement is the cyclic activated sludge (CASSBR), or sequencing batch reactor (SBR).
The technology permits bioreactor operation with considerably higher mixed liquor suspended solids (MLSS) concentration than CAS systems, which are limited by sludge settling.
The elevated biomass concentration in the MBR process allows for very effective removal of both soluble and particulate biodegradable materials at higher loading rates.
Membrane filters can be blinded with grease or abraded by suspended grit and lack a clarifier's flexibility to pass peak flows.
The technology has become increasingly popular for reliably pretreated waste streams and has gained wider acceptance where infiltration and inflow have been controlled, however, and the life-cycle costs have been steadily decreasing.
The small footprint of MBR systems, and the high quality effluent produced, make them particularly useful for water reuse applications.
[33] Conditions likely to create upsets include toxic chemicals and unusually high or low concentrations of organic waste BOD providing food for the bioreactor ecosystem.
Activated sludge recycle systems provide an integrated reservoir if upset conditions are detected in time for corrective action.
Others may find compatible agricultural wastes, or offer disposal incentives to septic tank pumpers during low use periods.
The reduced population surviving the starvation event may be unable to completely utilize available BOD when waste loads return to normal.
Smaller sewage treatment plants may experience dilution from cooling water discharges, major plumbing leaks, firefighting, or draining large swimming pools.
A similar problem occurs as BOD concentrations drop when low flow increases waste residence time within the secondary treatment bioreactor.
Secondary treatment ecosystems of college communities acclimated to waste loading fluctuations from student work/sleep cycles may have difficulty surviving school vacations.
Ecosystem populations of aerobic organisms increase until oxygen transfer limitations of the secondary treatment bioreactor are reached.
