Abstract
The importance of dissolved oxygen level in determining the platinum electrode potential in activated sludge has been clearly demonstrated by current-potential curves plotted at different oxygen concentrations. Tests have been carried out, in the laboratory and in full scale treatment plants,to define the relationship between the platinum electrode potential at equilibrium (Eh) and the dissolved oxygen [O2] concentration in the activated sludge. These two parameters obey a law of the form Eh = a + b log [O2]. The measured values of coefficents a and b differ widely to those found from the oxygen reduction reaction in water (a = 0.8 V at pH 7 and b = 15 mV per decade). Factors a and b mainly depend on the sludge loading, the aeration conditions and the sludge concentration. Using non–polished stationary platinum ring electrodes, the following values of a. were obtained (at pH between 7 and 7.6) : + 410 mV/NHE for sludge aerated for several hours without feeding, + 265 mV/NHE for over-aerated/low-loaded sludge (Cm = 0.2 kg BOD.kg MLVSS−1. day−1) and + 180 mV/NHE for high-loaded activated sludge in plug-flow system (Cm = 1 kg BOD.kg MLVSS−1.day −1). Factor b would seem to lie between 55 and 65 mV when the sludge is continuously aerated without feeding.At low loads with excess aeration, it lies between 70 and 90 mV. When the medium is slightly septic at low dissolved oxygen concentrations (insufficient daily aeration time, high sludge concentration or aerators shut down for too long periods), factor b increases and can reach 200 mV. In the same way, at high loads, factor b can become 150 mV. These results demonstrate the importance of dissolved oxygen concentration in the mechanisms which determine the metal electrode potentials in activated sludge. They also illustrate the role that other electroactive species play in the process. The type and concentration of these species depend on parameters such as the sludge loading, the overall oxygen supply, the aeration sequence and the sludge concentration.
Highlights
S eve r-a l hour-s wi t ho u t f e ed mg, ~ 26 5 r.'VIN HE fo l' ov erv a e r-atedvl ow-Load e d sl udg e
The equ i li br i um current den sity l Jo ) i s conven t i ona l ly det e rmi ned, a t pure t r a ns f er cond i ti ons, by r ec ord i ng current - vo l t age curve s at anod ic or cathod i c po ten tia l s ca ns l ow s pee ds and t hen pl ott i ng cur ves of th e l ogarithm of the c ur r ent density ( l og J) against t he app l i ed overpo t en tial (n)
LAfiORA IO RY AliD ON- 5 1 TE TEST PROCEDURES. In t he l abo r a tory, the e l ec t r odes a r e cl ean ed and t hen d i pped i nt o t he med i um appr oxi ma tel y one hour be fo restarti ng measu r ement
Summary
The jmp o r ~ ancpofd isso l ved o xy~ e n l e ve l 1n de t e r mi n i q th pplat inum el e c trod e poten t ia linacti va ted s l ud ge ha s be en clearly demo ns t rat ed by c u r ~ e n t - po tentlalc ur ve s p l o tt ed atdiffer en t oxygen co n c en t r a t on ". Fa c t o" s " and b rua in l y d e pend on t he s I udqe l oad i ng , t he a t' r a ~ lUn c ond i li o ns and thesl udge conc en t r a ti o n. T he ox idat ion r eduction potential /NHE (Vo l t s) the l oga r i t hm to a base of 10 t he d i s s ol ved oxygen co ncentra t io n Regardle s s of variation s due t o the d i s solved oxygen concentra tion , th e e l ectrode poten tia linac tivat ed sludge seems to be heav ily af fected by ot her electroactive sp ecies : a ccording t o NU8 8be r ger . I t is pos s i ble to obt ain r epea t abl e measuremen t s (to withi n ~ 10 mV) at s tab ili zed cond i tions by using e lect rodes in t he same materi al , of the same s hape and prepa r ed in th e s ame way
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