UASB Design
In general, there are two ways to design an UASB reactor

If input COD: 5,000 - 15,000 mg/l or more, the design method should be used based on Organic Loading rate, (OLR)
If input COD < 5000 mg/l, the design method should be calculated based on velocity.

Calculation UASB Tank Base on OLR
If input COD: 5,000 - 15,000 mg/l
with Organic loading rate ORL: 4 - 12 kg COD/m3.d
and Hydraulic retention time HRT: 4 - 12 h
COD treatment efficiency: E = (CODinput – CODoutput)/CODinput
In Calculation, Percent of COD removal is 75 - 85 %
Organic loading rate ORL = Q (CODinput – CODoutput) * 103
Volume of tank W = C * Q / OLR = (kg COD/m3 * m3/h) / (kg COD/m3.h)
C: concentration of COD in wastewater
Q: flow rate of wastewater
H (m) the height of tank can be calculated by: H = HS + HSe
The height of sludge layer Hs is: Hs = V * HRT
where Hs: the height of sludge layer area (main reactor)
and Hse: the height of sedimentation area
Where V = Velocity of flow 0.6 to 0.9 m/h
HRT = Hydraulic retention time (h)
In general, the height of sludge layer will be chosen in Table 1:
Table 1. Sludge Layer Height Selection COD input Sludge layer height < 3000 mg/l 3 – 5 m > 3000 mg/l 5 – 7 m Note: Sludge layer is longer than sludge bed layer
The height of setting area HSe ≥ 1.2 m and
The area surface of an UASB tank (m2): A = HRT * Q / H

Calculating an UASB Tank Based on Velocity
When input COD < 5,000 mg/l, using the method base on ORL is not effective in operation process because the granular sludge will be hardly formed. Therefore, the design criteria must be:

Up-flow velocity V Ј 0,5 m/h.
Hydraulic retention time HRT і 4 h
Chosen in table 1, the height of sludge is Hs = 3 – 5 m
The height of setting area HSe і 1.2 m

The volume of the UASB reactor: W = Q x HRT
The area of the UASB reactor: A = V / Q

GSL Separator Design
Slope of the separator bottom from 45 – 60o
Free surface in the aperture between the gas collectors: 15 – 20% of reactor area.
Height of separator from 1.5 – 2 m
The baffles to be installed beneath the gas domes should overlap the edge of the domes over a distance from 10 – 20 cm
Construct material: In the anaerobic conditions of an UASB reactor, there is a risk of corrosion in two main situations:

Some H2S gas can pass the GSL separator and accumulate above the water level in the top of the reactor. This will be oxidized to sulphate by oxygen in the air to form Sulphuric Acid that will in turn cause corrosion of both concrete and steel.
Below the water level: Calcium Oxide, (CaO), in concrete can be dissolve with by Carbon Dioxide, (CO2), in the liquid in low pH conditions.

To avoid these problems, the material used to construct the UASB reactor should be corrosion resistant, such as stainless steel or plastics, or be provided with proper surface coatings, (e.g. coated concrete rather than coated steel, plastic covered with impregnated hardwood for the settler, plastic fortified plywood, etc).
Operation
Operation criteria: The optimum pH range is from 6.6 to 7.6 The wastewater temperatures should not be < 5 °C because low temperatures can impede the hydrolysis rate of phase 1 and the activity of methanogenic bacteria. Therefore in winter season, methane gas may be needed to heat the wastewater to be treated in the reactor.
Always maintain the ratio of COD : N : P = 350 : 5 : 1 If there is a deficiency of some of these nutrients in the wastewater nutrient addition must be made to sustain the micro-organisms. Chemicals that are frequently used to add nutrients (N, P) are NH4H2PO4, KH2PO4, (NH4)2CO3...
Suspended solid (SS) can affect the anaerobic process in many ways:

Formation of scum layers and foaming due to the presence of insoluble components with floating properties, like fats and lipids.
Retarding or even completely obstructing the formation of sludge granules.
Entrapment of granular sludge in a layer of adsorbed insoluble matter and sometimes also falling apart (disintegration) of granular sludge.
A sudden and almost complete wash-out of the sludge present in reactor
Decline of the overall methanogenic activity of the sludge due to accumulation of SS

Therefore, the SS concentration in the feed to the reactor should not exceed 500 mg/l In phase 2 and 3 the pH will be reduced and the buffer capacity of wastewater may have to be increased to provide alkalinity of 1000 – 5000 mg/l CaCO3
Start-up: An UASB reactor requires a long time for start-up, e.g. from 2 – 3 weeks in good conditions (t > 20oC) and sometimes the start-up can take up to 3 – 4 months. In start-up process, hydraulic loading must be Ј 50% of the design hydraulic loading.
The start-up of the UASB reactor can be considered to be complete once a satisfactory performance of the system has been reached at its design load.


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