Sedimentation in detail

The sedi­men­ta­ti­on is a com­po­nent of the water fil­tra­ti­on in our recir­cu­la­ti­on sys­tem. But what exact­ly is it respon­si­ble for and how does it work? In the fol­lo­wing artic­le we will explain the com­po­nent in more detail.

The term „sedi­men­ta­ti­on“ is well known from was­te­wa­ter tre­at­ment plant tech­no­lo­gy. In a sedi­men­ta­ti­on, e.g. a sett­ling tank, the solids pre­sent in the water sink to the bot­tom due to their den­si­ty (hig­her than water). In the upper part of the sedi­men­ta­ti­on, clear water coll­ects (free from solids), which can be reu­sed or retur­ned to the ori­gi­nal pro­cess. The sedi­ment, an aque­ous sludge, can be coll­ec­ted and remo­ved at the bot­tom of the sedi­men­ta­ti­on. A sedi­men­ta­ti­on aims at water puri­fi­ca­ti­on or recovery.

The­re are two sources of solid par­tic­les in an aquacul­tu­re cir­cuit: 1. unea­ten feed pel­lets (fee­ding) and 2. the excre­ments of the fish (feces).

Both the feed pel­lets and the excre­ments still con­tain suf­fi­ci­ent nut­ri­ents to allow bac­te­ria to mul­ti­ply. Howe­ver, this effect is unde­si­ra­ble in an aquacul­tu­re plant, as too many of the wrong bac­te­ria can affect the health of the fish. The­r­e­fo­re, in aquacul­tu­re, the solids must be remo­ved from the sys­tem quick­ly and effi­ci­ent­ly. In recir­cu­la­ti­on sys­tems, the drum fil­ter coll­ects par­tic­les up to a cer­tain size through its fil­ter gau­ze, a very fine-mes­hed fil­ter net (approx. 40µm mesh size). The drum fil­ter needs regu­lar flus­hing of the fil­ter net in order to gua­ran­tee its func­tion per­ma­nent­ly. This rin­sing water, which is con­ta­mi­na­ted with the coll­ec­ted solid par­tic­les, is intro­du­ced into a sedi­men­ta­ti­on sys­tem in order to recy­cle it and redu­ce the water con­sump­ti­on of the plant.

In the SEAWATER Cube the sedi­men­ta­ti­on is a con­tai­ner with a distinc­ti­ve cone. The inlet is via a so-cal­led „Dort­mund­brun­nen“. In this the water is direc­ted ver­ti­cal­ly upwards. This crea­tes spe­cial flow con­di­ti­ons in which the par­tic­les con­tai­ned in the inflow water have more time to gather (coagu­la­te). As a result, even par­tic­les that are too light to sink on their own can coale­s­ce into lar­ger par­tic­les and then sink to the bot­tom due to their hig­her weight. A sludge is then for­med there.

Howe­ver, the bio­lo­gi­cal acti­vi­ty of the sludge poses is a fur­ther chall­enge. If suf­fi­ci­ent time is allo­wed, the bac­te­ria form fine gas bubbles (strong smel­ling fer­men­ta­ti­on gases), which turn the sludge at the bot­tom into so-cal­led bul­king sludge. Due to the gas bubbles, this sludge drifts upwards, which must be avo­ided at all cos­ts. The regu­lar rem­oval of the sludge coll­ec­ted in the cone of the tank by means of a pump is the­r­e­fo­re an important pro­cess step (seve­ral times a day). In addi­ti­on, regu­lar drai­ning and clea­ning of the tank is also man­da­to­ry (1–2 times a week).

In sum­ma­ry, sedi­men­ta­ti­on ser­ves to reco­ver rin­se water by sepa­ra­ting solids from the rin­se water of the drum fil­ter by sedi­men­ta­ti­on. The cla­ri­fied water can be fed back into the main cycle, saving a con­sidera­ble amount of water. In total, the SEAWATER cycle achie­ves to puri­fy 99% of the water in the plant its­elf. Only 1% of the water volu­me (~ 500 L) is lost per day through eva­po­ra­ti­on or flus­hing and has to be repla­ced. The fil­ter units thus make a decisi­ve con­tri­bu­ti­on to the sus­taina­bi­li­ty of our plant.

Refe­ren­ces:

— https://www.leiblein.de/produkte/schraegklaerer.html, 05.05.2020

— https://www.stadtentwaesserung-dresden.de/wasserwelten/wasser-wissen/dresdens-virtuelle-klaeranlage/, 05.05.2020

— Image source: SEAWATER Cubes