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Block 2 summary of AFI 31806
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Block 2 RAS systemLearning goals
• Know the different available solids removal processes
• Know the characteris:cs (pro and cons) of these techniques
• Know the effects on removal efficiency per process
• Being able to select the best solids removal process for applica:on in ‘new’ situa:ons.
Content
▪ Why remove solids?
▪ Solids source and characteris:cs
▪ Factors affec:ng solids produc:on
▪ Solids removal techniques.
Remove solids to – first treatment unit aPer fish tank
1. improve water quality – Fish Tank
a. Water affects fish performance and fish health.
b. Affects turbidity (interfere with fish finding food)
c. Solids seSles on fish egg causing heavy mortality.
2. improve opera:on other water treatment units.
a. Biofilter performance (low C/N Ra:o)
b. Disinfec:on
c. Prevents plugin of biofilters aera:ons columns, taps, screens, nozzles
d. Reduces system BOD load (O2 requirement) and CO2 produc:on.
e. Less bacterial respira:on due to solids diges:on
3. Concentrate solids for treatment or disposal.
a. Denitrifica:on (>C/N ra:o)
b. Floccula:on or diphosphatase process
c. Carbon source for detrivorous organism (circularity)
d. To mee regulatory requirement (disposal)
i. Suspended solids, BOD, nutrient
How to remove solids
1. As fast as possible
a. Solid breakdown easily in smaller par:cles
b. Smaller par:cles are difficult to remove
i. Solids <100 um are poorly seSleable
c. Prevent leaching of TAN,, phosphorus or DOM from solids
2. Which solids do we have?
a. Feed fines (frac:on should be as low as possible)
b. Uneaten feed
c. Feces
d. Sloughed biofilm
3. Factors affec:ng amount of solids
a. Frac:on of feed fines
i. Feed must be firm & withstand feed transport system
b. Frac:on of uneaten feed
i. Op:mal feeding rou:nes (sinking speed, distribu:on)
, ii. Prevent leaching feed par:cles (water stability)
c. Diges:bility of feed nutrients
i. Determines amount of feces
d. Amount of sloughed in biofilm
i. Biofilter type/reactor
Most important characteris:c by solids :
1. By size:
a. particle size distribution: Coulter counter, laser diffraction
b. serial filtration
2. by settling velocity (specific gravity)
3. by composition
i.e., sand, highly digestible uneaten feed, fecal waste,
bacteria flocs
Solids removal
Which technique
Selection bases on – Quality of water needed, size and desnsity partivles, and cost
We think for the fish in terms of growth, welfare and health
1) By gravity
a. Dual drain fish tank: Side wall drain (80-95% of the flow), and have center bottom drain
(5-20% of the flow)
, b. Settling basin: sedimentation from gravity
SeSling basin : pros and cons
Pros Cons
No moving parts Par:cles <100um not removed
Can be mounted at any orienta:on Anaerobic or anoxic condi:ons
Minimum supervision Nutrient leaching
Minimum maintenance Resuspensions through gas forma:on or escaped fish
Almost no head loss In general large surface area
Considered not to be effec:ve in RAS
Not con:nuous
c. Plate or tube settlers
, Design characteris:cs
Plate or tube settlers: Pros and cons
Pros Cons
No moving parts Par:cles <75um bit removed
Can be mounted at any orienta:on Anaerobic or anoxic condi:ons
Minimum supervision Nutrient leaching
Resuspension through gas forma:on
Periodic draining and cleaning
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