Aerated Lagoons/Ponds

Typically employed for treating municipal wastewaters, pulp and paper effluents, etc

Usually designed as CFSTRs without recycle

Typical Process Flowsheet

Mass Balance Equations

Initially, only consider mass balances on viable biomass (X) and biodegradable COD (S)

Biomass Mass Balance

Substrate Mass Balance

Solving the Equations

The soluble COD in the lagoon effluent will equal the sum of the residual biodegradable COD + the non-biodegradable soluble COD in the influent

The total concentration of VSS in the lagoon effluent includes viable biomass + endogenous decay products + non-biodegradable VSS that is present in the influent

The TSS concentration in the lagoon effluent includes:

-  FSS that are present in the influent

-  FSS that are present in the biological solids that are generated in the lagoon

o  Typically assume a volatile fraction of biomass

Sludge Age

A measure of the average time that biomass spends in the system

An example:


Biomass Washout

A condition where biomass is washed out more rapidly that it can grow

From previous biomass mass balance:

For a fixed reactor volume, as flow increases, substrate (S) approaches the influent concentration and biomass (X) approaches 0

Qm is the critical flow when S = So and X = 0

Rearranging the previous equation:

Design of processes should maintain Q sufficiently greater than Qm to ensure that washout will not occur.


Design

Typical materials of construction:

·  concrete

·  earthen with liners

Aeration and Mixing:

·  none (anaerobic)

·  wind (facultative)

·  mechanical aerators or diffused aeration (aerobic)

details of design later

Sedimentation of effluent is required

- often an unmixed pond is employed

Some typical design and operating parameters

Parameter / Anaerobic / Facultative / Aerated
Residence time (days) / 20-50 / 7-30 / 3-10
BOD Loading (kg BOD/ha-d) / <500 / <50 / NA*
BOD Removal (%) / <80% / 80-90% / 85-99%
Effluent -BOD (mg/L) / variable / <30 / <30
Depth (m) / >2 / 1-2 / 2-7
Design Hydraulics / PFR
CFSTR / PFR / CFSTR

* A wide range of loadings is possible depending upon level of aeration

Design Process

An iterative approach is required

·  assume volume and depth

·  calculate surface area

·  estimate operating temperature

·  calculate effluent concentrations

·  check against required values

·  adjust volume

1.) Define: Influent characteristics (flow, w/w characterization)

k, Ks, Y, kd - biokinetic coefficients at 20o C

S - treatment efficiency

Volume and depth → Surface Area

2.) Estimate operating temperature

The Mancini and Barnhart equation is an empirical relationship that has been found to be adequate

3.) Adjust temperature sensitive coefficients

4.) Solve design equations to calculate effluent concentrations

5.) Iterate

6.) Calculate solids production

Required for:

-  aeration system design (viable biomass + cell debris)

-  settling pond design (TSS)

7.) Calculate oxygen requirements

Oxygen required by biomass to oxidize organics that exert bCOD

A minimum of 2 mg/L is usually required

O2,R = oxygen requirements, kg O2/day

8.) Design aeration system

see later

9.) Design sedimentation pond

An Example: