Water tools

Disinfection contactor: pipeline dispersion and serpentine basin t10

A contactor that lets some water through early gives that water less than its CT. Dispersion measures the spread; the higher the log goal, the less of it you can afford.

Log inactivation goal

Flowm³/d

Design residence timemin

Pipeline lengthm

Friction factor

Basin length to width ratioAt least 20, preferably 40, for baffled reuse contactors.

Coefficient of ideality3 for a good design, 7 for a typical older basin, up to 15.

Goal

Allowable dispersion number for the goal

0.0060

Pipeline

Pipe diameter

0.81m

Velocity

0.56m/s

Dispersion number

0.00010

Within the allowance: close to plug flow.

Serpentine basin

Dispersion number

0.0105

t10 / mean residence time

0.73

t10 for credit

21.9min

How it works

Pipeline: diameter from the residence time, dispersion by the field-fitted relation:

D = \sqrt{\frac{4 Q τ}{π L}} d = 89, 500 f^{3.6} {(\frac{D}{L})}^{0.859}

(1)

Serpentine basin: dispersion from the aspect ratio and the ideality coefficient, and the t10 ratio from the aspect ratio:

d = \frac{0.14 C_{i}}{β_{L}} \frac{t_{10}}{τ} = 0.198 \ln β_{L} - 0.002

(2)

where

$D$: pipe diameter, m
$Q$: flow, m³/s
$τ$: residence time, s
$L$: pipe length, m
$d$: dispersion number
$f$: friction factor
$C_{i}$: coefficient of ideality
$β_{L}$: basin length to width ratio
$t_{10}$: time for the first tenth of a tracer to appear

The allowable dispersion is where a first-order process falls 5% short of its goal: about 0.03 at 1 log, 0.006 at 4 log. Regulators credit the t10, the time for the first tenth of a tracer to appear, which is conservative against a full residence-time model. Keep the flow-path width constant through 180 degree turns and add diffuser walls at inlet and outlet; up to 40% of a badly baffled tank can be dead volume. The t10 relation holds for aspect ratios of about 5 to 100.

Disinfection contactor: pipeline dispersion and serpentine basin t10

How it works

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