Application of the activated sludge model to aerated lagoons

G.V.R Marais,M.C Wentzel,G.A Ekama

doi:10.4314/wsa.v43i2.08

Abstract

The different kinds of aerated lagoons, which exclude anaerobic pre-treatment ponds, are described and the design approach for aerated lagoons is explained. This hinges around ensuring that the 1st lagoon is suspension mixed and the second and any additional are facultative. Selection of the retention time for the 1st lagoon is important to ensure complete utilization of the influent biodegradable organics. Minimum retention times to achieve this at 14 degreesC and 22 degreesC were determined with the general activated sludge kinetic simulation model for (i) readily biodegradable soluble organics (BSO) only, (ii) slowly biodegradable particulate organics (BPO) only, (iii) real municipal wastewater (20% BSO and 80% BPO) and (iv) real municipal wastewater with 5% OHO active VSS mass seed. The minimum hydraulic retention times for these four cases are: at 14 degreesC 1.3, 3.0, 2.0 and 1.5 d, respectively, and at 22 degreesC 0.3, 2.0, 1.2 and 1.0 d, respectively. From a comparison of the simulation results with the steady-state model calculations, washout of OHOs takes place at about 75% of these retention times. Approximate equations to estimate the power requirements for aeration by mechanical surface aerators and mixing are given. These equations are combined with those of the steady-state activated sludge lagoon model for calculating the oxygen requirements and the aeration power density (W/m3) in each lagoon. With these equations, it is shown that influent COD concentration needs to be between an upper and lower limit band to ensure that the 1st lagoon is suspension mixed and the second lagoon is facultative. This COD concentration band decreases as the influent flow increases. The important conclusion arising from this is that if the aerated lagoon system is applied for small rural communities, where land for these large systems is likely to be available, then additional mixing energy above that for aeration will need to be provided to ensure that the 1st lagoon is suspension mixed - this additional aeration cost makes it unlikely that aerated lagoons will be applied for municipal wastewater treatment. Matching mixing and aeration power requirements for industrial organic wastewaters is easier because these usually are significantly stronger than municipal wastewaters.

Highlights

Oxidation pondThe lagoon system of wastewater treatment covers a spectrum of clearly definable systems differentiated by the degree of mixing and the method of oxygenation
Even if biodegradation of the influent biodegradable organics is virtually complete in the 1st lagoon, the COD removal is still low
This is because the activated sludge (AS) formed in the 1st lagoon is part of the unfiltered effluent COD (Eq 9) because the lagoon is suspension mixed

Summary

INTRODUCTION

The lagoon system of wastewater treatment covers a spectrum of clearly definable systems differentiated by the degree of mixing and the method of oxygenation. The filtered effluent COD is very low because (depending on the wastewater type and retention time) most of the influent biodegradable soluble organics (BSO) are utilized and transformed to OHO VSS mass (Eqs 5 and 6). Like Kv, this does not influence the design of the 1st lagoon very much because (i) a retention time is selected so that the residual soluble and particulate (enmeshed with the VSS) biodegradable organics are very low and (ii) the design approach is based on the carbonaceous OD which is not strongly influenced by the residual biodegradable organics concentration, especially if the influent BOD5 is high.

DESIGN OPTIMIZATION

Distillery

DESIGN EXAMPLE

Findings

CONCLUSION