Aerobic wastewater treatment under high and varying temperatures : thermophilic process performance and effluent quality

Abstract

Tuottavassa teollisuudessa, esimerkiksi paperin ja massan valmistuksessa, muodostuvat kuumat prosessi- ja jätevedet tyypillisesti viilennetään ennen niiden biologista käsittelyä. Juhani Suvilammen väitöstutkimus osoittaa, että jätevesiä voidaan käsitellä myös korkeissa lämpötiloissa, mistä on monia etuja.Tutkimuksen tuloksia voidaan hyödyntää kuumien jätevesien biologisessa aerobisessa käsittelyssä ja erityisesti metsäteollisuuden vesikiertoja suljettaessa. Tutkittu prosessi on aikaisempaa yksinkertaisempi, koska lämmönvaihtimien tarve pienenee. Käsittely on siten kustannustehokkaampi ja toiminnaltaan varmempi. Lämpötilan noustessa reaktionopeudet kasvavat, joten prosessi voidaan toteuttaa myös pienemmässä reaktorissa. Pienemmät reaktorit puolestaan merkitsevät alhaisempia investointeja. Prosessi tuottaa myös vähemmän lietettä, mikä lisää sen kustannustehokkuutta. Kuumien jätevesien käsittelyä on kuitenkin täydennettävä hienoaineksen poistamisella, jotta saavutettaisiin sama laatu kuin alhaisemmissa lämpötiloissa.

Industries, such as the pulp and paper industry, generate high-temperature process waters and wastewaters. Biological treatment at high temperatures may be an attractive option for the treatment of hot or concentrated wastewaters. The objective of this study was to evaluate the feasibility of thermophilic aerobic wastewater treatment in laboratory experiments, in which thermophilic processes were compared with mesophilic processes. A combined thermophilic-mesophilic treatment was used to improve thermophilic effluent quality. Also the performance of a pilot thermophilic aerobic suspended carrier biofilm process (SCBP) treating groundwood mill circulation water (GWM) was studied on mill premises. Thermophilic and mesophilic activated sludge processes (ASPs) were compared under different hydraulic retention times (HRTs) 18-8 h and volumetric loading rates (VLRs) 2.5-6 kg CODfilt m-3d-1. In thermophilic ASPs total COD (CODtot) and GFA-filtered COD (CODfilt) removals were lower (65-75%) than in the mesophilic (85-90%). Both ASPs gave similar CODsol (0.45 ?m-filtrated COD) removals (90%), whereas effluents from the thermophilic ASPs had notably higher CODcol (calculated COD between 0.45-1.6 ?m) values (460?170 mg l-1) than mesophilic effluents (6?5 mg l-1). The increased CODcol was due to the high density of dispersed particles, which were unable to aggregate and settle under thermophilic conditions. These bacteria were mostly non-viable gram-positive, whereas small (<1 ?m) thermophilic gram-negative bacteria were dominant in the settled flocs. Combined thermophilic-mesophilic treatment had CODfilt removals of 80-85% with HRTs from 36 to 18 h. Thermophilic treatment removed CODsol (90%), whereas mesophilic post-treatment removed free bacteria from the thermophilic effluent, measured as 90-100% CODcol removals. Between HRTs of 12-18 h the thermophilic ASP and SCBP removed 60? 13% and 62?7% of CODfilt, respectively, whereas with HRT of 7-8 h the removals were 48?1% and 69?4%. A two-stage thermophilic SCBP produced 49-77% dissolved organic carbon (DOC) removals at VLRs of 3-14 kg CODfilt m-3d-1 and HRTs of 2-8 h, whereas the single reactor produced 40-67% DOC removals at VLRs of 7-28 kg CODfilt m-3d-1 and HRTs of 1-4 h. The two-stage thermophilic process was particularly resistant to process upsets. In conclusion, thermophilic aerobic wastewater treatment proved operable under varying parameters and yielded CODsol removals comparable to those obtained from mesophilic treatment. Increased temperature reduced CODtot and CODfilt removals.