Water Treatment Media Glossary: 59 Terms Explained

Media and packing

The physical elements that sit inside a tank, and the geometry that describes them.

Specific surface aream²/m³: The wetted surface a packing offers per cubic metre of installed volume. It is the single number that most directly sets how much biofilm a reactor can hold or how fine a settle a lamella can achieve — but it is only useful alongside the void ratio, because surface you cannot flow past does nothing. See MBBR carriers.
Void ratio / porosity%: The proportion of the installed volume that is open space rather than material. High specific surface area and high void ratio pull against each other: adding internal walls raises surface area but narrows the passages, and past a point the media blocks rather than treats.
Tube settler mediainclined tube packing: Modules of small-diameter tubes installed at an angle in a clarifier so that particles only have to fall a short, defined distance before they reach a surface. Also called inclined tube packing or honeycomb tube media. See tube settler media.
Inclined plate medialamella plate: The flat-sheet equivalent of tube settler media: parallel plates at the same installation angle, forming the same shallow settling cells from a different geometry. The trade-off against tubes is described in tube settler vs lamella clarifier.
Installation angleusually 60°: The angle of the tubes or plates from horizontal. Steep enough that settled sludge slides down and out rather than accumulating, shallow enough that the settling distance stays short. Sixty degrees is the conventional compromise for municipal sludge.
MBBR carriermoving bed biofilm reactor media: A small moulded plastic element that moves freely in an aerated tank while biofilm grows on its protected internal surface. The carrier does not separate solids — that still happens downstream. See what is MBBR and MBBR moving bed media.
Fill fractiondosing ratio / fill ratio: The percentage of reactor volume occupied by carriers at rest. It is a design output, derived from the load to be removed, not a figure to pick from a table. The method is set out in MBBR media calculation.
Random packingdumped packing: Individual shaped elements tipped into a tower in no particular order. Cheap to install and tolerant of fouling, with a pressure drop that depends on how the pieces happened to land. See Pall ring packing.
Structured packingordered packing: Packing built from repeated geometric units so the flow path is predictable. Lower pressure drop per unit of surface area than random packing, but far less tolerant of solids.
Pall ring: A random packing shape with a cylindrical wall and internal tongues that redistribute the falling liquid and re-expose fresh surface. Used in degassing and scrubbing rather than in biological treatment. See Pall ring packing.
Suspended ball media: A hollow or multi-faceted plastic sphere used as a bio-carrier or as a surface-cover ball. Buoyancy, diameter and whether the sphere is open or closed decide which duty it can do. See suspended ball media.
Hanging mediarope, elastic, combined packing: Carriers suspended on a frame rather than moving freely, as in elastic packing, combined packing and biological rope media. Suited to low-energy or anaerobic tanks where mechanical movement is not available.
Fibre ball media: A ball of fine synthetic fibres used as a filter medium rather than as a biofilm carrier. The fibre diameter and the ball density together set the capture behaviour. See fibre ball filter media.
Filter nozzlestrainer nozzle, water cap: The distribution element threaded into a filter plate, forming one flow path between the plenum and the media bed. Slot width and head shape decide what it retains and how evenly it spreads backwash. See ABS filter nozzle.
Filter platenozzle plate: The horizontal plate that carries the nozzles and separates the media bed from the plenum below. Plate type and tapping thread are decided together with the nozzle, not separately.
Plenum: The void beneath a filter plate through which filtered water leaves and backwash water and air are distributed. An uneven plenum produces uneven backwashing no matter how good the nozzles are.
Support gravel bed: Graded layers of gravel beneath a filter medium, once universal and now often replaced by a nozzle plate. Its job was to stop the medium from passing into the underdrain.

Settling and clarification

What happens between the water arriving and the sludge leaving.

Surface loading rateoverflow rate, m³/m²·h: Flow divided by the horizontal surface area of the settling zone. It is the fundamental sizing number for any clarifier: a particle whose settling velocity is below the surface loading rate will not be captured, regardless of how long the tank is.
Hydraulic retention timeHRT: Tank volume divided by flow rate. It tells you how long the water stays, not how well it is treated — two tanks with identical HRT can perform completely differently depending on mixing and short-circuiting.
Short-circuiting: Water taking a faster path through a tank than the design intends, so the effective retention time is shorter than the calculated one. Baffles and correctly installed media exist largely to prevent it.
Coagulation and flocculation: Coagulation destabilises particles so they can stick together; flocculation gives them gentle, sustained contact so the aggregates grow large enough to settle. They are separate stages with different mixing requirements, and confusing them is one of the most common design errors.
Flocculatorflocculation basin: The stage where floc is built up. Plate or baffle geometry inside it sets both the mixing energy and the number of elements that fit — the arithmetic for the plate count is in the flocculator plate calculator.
Lamellaplate settler: One inclined settling surface within a stack. The stack achieves a large effective settling area in a small footprint by dividing the depth into many shallow cells.
Dissolved air flotationDAF: A separation stage that lifts particles to the surface with dissolved air bubbles rather than letting them sink. Chosen where the solids are buoyant or where the floc is too light to settle reliably.
Sludge blanket: The layer of accumulated solids in a clarifier through which incoming water rises. A stable blanket filters as well as settles; an unstable one either washes out or turns septic.

Aeration

Getting oxygen into water, and the numbers used to describe how well it is done.

Diffuseraerator: The device that breaks air into bubbles below the water surface. Membrane diffusers dominate because a perforated elastic membrane produces finer bubbles than a fixed orifice and resists fouling better. Disc and tube formats are compared in disc diffuser vs tube diffuser.
Fine bubble1–3 mm: Bubbles in the low-millimetre range, which give a large transfer surface per unit of air volume. The trade-off is higher head loss through the diffuser and a membrane that must be kept open.
Oxygen transfer efficiencyOTE: The fraction of the oxygen in the supplied air that actually dissolves. It rises steeply with water depth, which is why a deeper basin beats a shallower one more convincingly than a better diffuser does.
Standard oxygen transfer rateSOTR: The mass of oxygen a diffuser system transfers per hour under standard reference conditions — clean water, defined temperature and zero dissolved oxygen. It is a benchmark for comparing diffusers, not a prediction of plant performance.
Air scour: Using air alone, or air with a small water flow, to break up a filter bed during backwash. It is more effective than water alone at dislodging attached deposits, and harsher on the medium.
Membrane opening rate: The minimum airflow at which a diffuser membrane lifts off its support and opens fully. Run below it and the membrane stays partly closed, the bubbles coarsen and fouling starts on the underside.
Turndown: How far airflow can be reduced before performance degrades. Long diffusers generally offer a wider turndown than short ones, which matters in plants whose load swings strongly between day and night.

Biological process

The configurations and the operating parameters that define them.

Biofilm: A community of micro-organisms attached to a surface and held in a matrix they produce themselves. Attached growth tolerates load shocks and low temperatures better than suspended growth, which is why carriers are used to upgrade overloaded plants.
Activated sludgesuspended growth: The conventional process in which biomass grows as flocs suspended in the tank and is separated downstream in a clarifier. Compared with attached growth in MBBR vs activated sludge.
Membrane bioreactorMBR: Activated sludge with a membrane replacing the clarifier. It is a separation technology, not a biofilm one — see MBBR vs MBR.
Sequencing batch reactorSBR: A single tank that fills, reacts, settles and decants in timed cycles instead of using separate tanks. Suited to small and intermittent flows, and to plants that need flexibility more than throughput.
Anaerobic–anoxic–oxicA²O: A three-stage arrangement that removes phosphorus and nitrogen as well as organic matter by exposing the biomass to alternating conditions, with recycle streams connecting the stages.
Nitrification: The biological conversion of ammonia to nitrate, carried out by slow-growing organisms — which is why it needs a longer sludge age and more oxygen than carbon removal alone.
Denitrification: The conversion of nitrate to nitrogen gas under anoxic conditions. It needs a carbon source and a zone with no dissolved oxygen, and it is the step that actually removes nitrogen from the plant.
Sludge ageSRT, solids retention time: How long, on average, a solid particle stays in the system. Set it too low and slow-growing nitrifiers wash out; set it too high and the sludge becomes over-oxidised and hard to settle.
Food-to-microorganism ratioF/M: The mass of incoming substrate per mass of biomass per day. It is the classic lever for moving a plant between high-rate and low-rate operation.
Sludge volume indexSVI: A settling test that indicates whether sludge will separate well or bulk. High values warn of filamentous growth before it shows up as solids carry-over.
Hydraulic and organic shock: A sudden change in flow or load. Attached-growth systems ride these out better than suspended-growth ones because the biofilm is not washed downstream with the flow.

Water quality parameters

What the numbers on a laboratory report actually measure.

Biochemical oxygen demandBOD: The oxygen consumed by micro-organisms breaking down organic matter in a sample over a fixed period. It measures the biodegradable fraction — not total organic content.
Chemical oxygen demandCOD: The oxygen needed to oxidise all the chemically oxidisable matter in a sample. It is always at least as high as BOD, and the gap between the two indicates how much of the organic load is not readily biodegradable.
Total suspended solidsTSS, SS: The mass of particulate matter retained by a filter under a defined test. It is the parameter most directly affected by clarification and media performance.
TurbidityNTU: An optical measure of how much light is scattered by particles in the water. Cheap to measure continuously, which makes it the routine substitute for gravimetric suspended solids.
AmmoniaNH₃-N, NH₄-N: Nitrogen present as ammonia or ammonium, reported as nitrogen. It is toxic to fish, exerts an oxygen demand in the receiving water and is the target of nitrification.
Dissolved oxygenDO: The oxygen actually in solution. The controlling variable in an aerated tank: too little and the biology stalls, too much and energy is being wasted.
Mixed liquor suspended solidsMLSS: The concentration of solids in the aerated tank. The upper limit is set not by biology but by what the clarifier can return.
Head loss: The pressure lost as water passes through a medium, packing or diffuser. It is what you pay in pumping energy for the benefit of surface area or fine bubbles.
HS code: The customs classification that determines duty and import documentation for a shipment. For treatment media it is assigned by material and function, and it must be confirmed before the goods leave. See wastewater media HS code.

Materials

The abbreviations that appear in every specification sheet.

PolypropylenePP: The default material for settling media and cheap carriers: light, chemically resistant and weldable. It is also the material whose service life is most sensitive to UV exposure and to warm, oxidising service.
Ethylene–propylene copolymerE-P copolymer: A copolymer grade used where polypropylene alone would become brittle. It costs more and lasts longer, which is a whole-life argument rather than a first-cost one.
Acrylonitrile butadiene styreneABS: A rigid engineering plastic used for pipework, nozzles and aeration fittings. It machines and threads well and holds dimension under load, which is why filter plates are tapped for ABS rather than for softer plastics.
Ethylene propylene diene monomerEPDM: The standard membrane elastomer for municipal aeration. It tolerates normal wastewater chemistry and repeated flexing, and it is attacked by oils and solvents — which is when silicone is specified instead.
Fibre-reinforced plasticFRP, GRP: Resin reinforced with glass fibre. Used where stiffness and corrosion resistance matter more than weight, such as structural troughs and heavy-duty settling modules.
High-density polyethyleneHDPE: A tough, chemically inert plastic used for carriers and liners. Its density relative to water decides whether a carrier fluidises readily or sinks and stacks.
UV stabilisation: Additives that slow sunlight degradation. Relevant to any media stored outdoors before installation, and to surface-cover balls that spend their life exposed.

Water treatment media glossary