Anaerobic on-site wastewater treatment at low temperatures

Abstract

Sari Luostarinen tutki väitöstyössään hajautettua anaerobista jätevesien käsittelyä alhaisissa lämpötiloissa (10-20 °C). Anaerobinen käsittely mahdollistaa sekä jätemateriaalin stabiloinnin, uusiutuvan energian tuotannon metaanina ja/tai vetynä että arvokkaiden yhdisteiden, kuten ravinteiden, uudelleenkäytön. Luostarisen mukaan käsittely voidaan toteuttaa hajautettuna kiinteistö- tai kyläkohtaisissa prosesseissa. Mikäli ravinteiden uudelleenkäyttö ei ole mahdollista, esimerkiksi hygieniamääräyksien vuoksi, ja ravinteiden poistoa vaaditaan, se voidaan toteuttaa aerobisessa jälkikäsittelyprosessissa. Luostarisen tutkimuksessa kaksivaiheinen anaerobinen ylösvirtaussaostuskaivo (UASB-saostuskaivo) havaittiin soveltuvaksi synteettisen käymäläjäteveden, maitohuoneen jäteveden sekä käymäläjäteveden ja keittiöjätteen käsittelyyn. – UASB-saostuskaivon avulla poistettiin yli 90 % kokonaiskemiallisesta hapenkulutuksesta (COD), 98 % kiintoaineesta, 70 % liuenneesta COD:sta sekä yli 90 % biologisesta hapen kulutuksesta riippumatta käytetystä lämpötilasta, Luostarinen kertoo tutkimuksensa tuloksista.

Anaerobic on-site wastewater treatment at low temperaturesAnaerobic treatment stabilises the treated waste(water), and enables production of renewable energy (methane, hydrogen), and preservation of valuable resources (nutrients). Decentralised, on-site application of anaerobic treatment for communities or individual households may thus provide combined waste(water) management, energy production, and nutrient recovery. Two-phased UASB-septic tanks were feasible for treatment of synthetic black water, dairy parlour wastewater, and a mixture of black water and kitchen waste at low temperatures of 10–20 °C with respective removals of total, suspended solids, and dissolved chemical oxygen demand (CODt, CODss, CODdis), and biological oxygen demand (BOD7) being over 90, 98, 70, and over 90 %. In pilot studies with single-phased UASB-septic tanks and more concentrated black water, 65 % of CODt and 70–80 % of CODss were removed, while CODdis removal was negative during 1st-year-operation and improved to over 50 % over time. At lower temperatures, COD removal was more due to settling and accumulation, while at higher temperatures, biological activity increased and more COD was converted to methane, the highest conversion being 44 % (mixture of black water and kitchen waste, 20 °C). Black water from conventional flush toilets may be treated in single-phased UASB-septic tank especially with nitrogen removing post-treatment, while two-phased anaerobic process is recommended for low-temperature treatment of the other studied wastewaters. High removal of suspended solids in phase 1 resulted in sludge bed growth, and sufficient sludge retention time for stabilisation has to be provided. With nutrient removal requirement, post-treatment of anaerobically treated dairy parlour wastewater (10 °C) and mixture of black water and kitchen waste (20 °C) in intermittently aerated moving bed biofilm reactors resulted in 50–60 % nitrogen and 40–70 % CODt removals. Complete nitrification was achieved with 2.0–3.5 mg dissolved oxygen/l and sufficiently long aeration period, while denitrification suffered from carbon shortage. Carbon addition increased nitrogen removal instantly to 83 %, wherefore the anaerobic process should be optimised to retain sufficient carbon for denitrification. The combination of two-phased UASB-septic tank and moving bed biofilm reactor removed over 92 % of CODt, 88 % of CODdis, 99 % of BOD7, 80 % of phosphorous, and 65-70 % of nitrogen at 10-20 °C, thus proving an efficient combination for on-site wastewater treatment. Mixture of kitchen waste and black water also showed potential for fermentative hydrogen production due to readily soluble carbohydrates for hydrogen producers, while synthetic black water seemed less suitable with its high solid/readily soluble organic matter ratio.