Aerated lagoon treatment of piggery wastes: Kinetics of carbon removal

Parallel aerated lagoons were operated for almost 2 years at various organic loads. The feed consisted of wastewater from a large piggery-farm pretreated in anaerobic lagoons. First-order kinetic models were developed for substrate removal and biomass generation. The constants from these models were: removal rate k = 0·0014 litre day⁻¹ mg⁻¹, biomass yield Y = 0·5 and decay coefficient k_d = 0·02 day⁻¹. It was found that lagoon performance should be interpreted against volumetric organic load rather than against hydraulic residence time or surface load. A load removal kinetic expression of the type S/S₀ = exp(− K/L) is proposed. The lagoon data interpreted according to this formula showed that the organics removal occureed sequentially. Two rates were defined. In the case of BOD removal there was a high rate (K₁ = 0·6 kgm⁻³ day⁻¹) down to a load of L = 0·6 kg m⁻³ day⁻¹; and a low rate period $\text{(K}{}_2\text{= 0·06 kg m}{}^{\mathrm{-3}}\text{day}{}^{\mathrm{-1}}\text{) below L = 0·6 kg m}{}^{\mathrm{-3}}\text{day}{}^{\mathrm{-1}}$. The turning point occurred at about HRT (hydraulic residence time) =3·2– 4·0 day. The proposed load interpretation allows for optimising the lagoon design into a series of high- and low-rate reactors, where the total volume will always be lower than the volume of one lagoon for the same treatment level.