טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentKrasnits Eli
SubjectPHA - Driven Denitrification in Municipal Wastewater
DepartmentDepartment of Civil and Environmental Engineering
Supervisor Professor Emeritus Michal Green
Full Thesis text - in Hebrew Full thesis text - Hebrew Version


Abstract

Removal of nitrogen compounds from wastewater is usually performed by nitrification followed by denitrification. The combination of these two processes has some drawbacks, mainly: 1) oxygen is essential for nitrification and inhibits denitrification and 2) removal of organics prior to nitrification, in order to achieve efficient nitrification, results in electron donor deficiency for the subsequent denitrification.

Intracellular carbon storage of polyhydroxyalkenoates (PHA) in heterotrophic bacteria is a well-known phenomenon and was found to be a major metabolic pathway in conventional wastewater treatment plants.

Until now, PHA driven denitrification systems were studied mainly with synthetic based wastewater and only to some extent with real wastewater. The previous studies provided information regarding COD, nitrogen and phosphorus removal potential, however, the composition and stoichiometry of storage biopolymers were only investigated using synthetic wastewaters such as acetate and propionate.

This research investigated PHA driven denitrification in municipal wastewater, using a biofilm SBR performing alternately anaerobic carbon storage and denitrification as part of potential two-sludge system for removal of organics, nitrogen and phosphorus from wastewater (nitrification reactor was not included in this work).

The work included three main chapters:

1. Investigation of denitrification potential and storage compounds composition and degradation kinetics on real wastewater and in comparison to acetate based synthetic wastewater.

2. Effect of the denitrification stage duration on PHA storage and consumption and on denitrification efficiency.

3. The contribution of wastewater suspended solids to PHA storage and denitrification potential.

The main findings of this work are:

- Anaerobic storage followed by PHA based denitrification with municipal wastewater, yielded high nitrate removal of 39-52mgN/L, with a low COD requirement (COD/N ratios of 4.6-5.3g/g), and with daily rates similar to those of extensive treatment systems (0.24-0.31gN/L*d).

- With the acetate based wastewater, 95% of the total COD removed was stored as PHA, while with real wastewaters less than 50% of the total COD was stored as PHA.

- PHA degradation kinetics showed a good fit to 1st order reaction regardless of the denitrification stage duration.

- Reducing the denitrification duration from 3 to 2 hours resulted in increased sludge PHA content with a corresponding increase in the PHA degradation rate. As a result, the daily denitrification rates increased by 30%-34%.

- During feed with wastewater, suspended solids contributed about 50% of the reducing power for denitrification while PHA provided the rest.