Page 1


Wastewater Engineering Lecturer:

Tower 1/ Aras

, Room

Ext. Pond Systems DA/RH

Learning Outcome At the end of Week 9 students should be able to - describe oxidation ponds

PONDS 1. AERATED PONDS BOD removal: Basis of design, : 3-6 days S = Ks(1+kd)/[(Yk-kd)-1] Process Efficiency= [(So-S)/So] 100 Based on 1st Order Removal Function S/So = 1/[1+k(V/Q)] Effluent Characteristics: Soluble BOD5 and SS Concentrations: X = Y(So-S)/(1+Kd) Oxygen Requirements: 0.7-1.4 times the amount of BOD5 removed kg O2/day = [Q(So-S)/0.68] – 1.42Px Px = amount of biological solids wasted per day

PONDS 2. MARAIS THEOREM FOR PONDS IN SERIES A mass balance of BOD around mth. pond: Lm/Lm-1 = 1/(1+k1t) For the whole series of n ponds: Le/Li = 1/(1+k1t)n If =total retention time in the series of ponds:  = nt In the limit as t0 (and if  is to remain the same as n) Le= Ltm 1 . Li t0 (1+k1t)/t i.e. Le/Li = e-k  plug flow


Most efficient hydraulics regime Series of small ponds more efficient

than big

ponds Marais Theorem – max efficiency if retention time in each pond is the same

PONDS 3. REACTOR WITHOUT RECYCLE A mass balance is a function of time: Accumulation = Inflow – Outflow + Utilisation Solution: Unsteady: C = (Q/V)(Co/)(1-e-t) + Coet where t : C = (Q/V)(Co/) = Co/[1+K(Q/V)] Steady: dc/dt = 0 0 = QCo – QC + VKC C = Co/[1+K(Q/V)] where C = concentration of effluent Co = concentration of the influent

PONDS 4. MATURATION PONDS ďƒ­For a maturation pond, pathogenic organisms are removed: Ne = Nt /(1 + kbt) kb = 1st order Rate Constant for Faecal Coliform removal per day = 2.6(1.10)T-20 ďƒ­When facultative pond followed by maturation pond: Ne = Nt /[(1 + kbtf) (1 + kbtm)] where tf, tm are detention times in the facultative & maturation ponds, respectively

PONDS 5. THE NON-IDEAL FLOW DESIGN PROCEDURE (Wehner & Wilhelm) Accurate Formula: Ce = 4a.e(d/2) . Ci (1+a)2.e(a/2d) – (1-a)2.e-(a/x) Modified Formula: Ce = 4a.e([1-a]/[2d]) Ci (1+a)2 Approximate Formula: Ce = e(-kt) Ci

Where t = mean detention time, a = (1+4KTD)0.5, d= D/UL (dispersion no.), D = axial dispersion coefficient, U = fluid velocity, L = length of fluid towel



ECW542 Pond Systems Lecturer: DA/RH At the end of Week 9 students should be able to - describe oxidation ponds Learning Outcome Soluble BOD...