Removal of MBBR Biofilm Solids by Salsnes Filter Fine Mesh Sieves

Abstract

Biological wastewater treatment is often used in conjunction with primary treatment to reduce the constituents in wastewater. It is normally necessary to separate the biomass from the treated wastewater in order to meet the effluent discharge standards. Moving Bed Biofilm Reactor (MBBR) is a biofilm process where plastic carriers carrying the biomass are moving along with the wastewater and typically operating with low concentration of suspended solids in pure biofilm systems. Salsnes Filters AS (SF) is a Norwegian company specializing in design, manufacturing and supply of patented fine mesh filter machines for treatment of primary wastewater and looking to expand use of their products for the separation of biofilm solids following biofilm processes.

This study describes an overall assessment of the performance of SF sieve cloths for separation of biofilm solids with and without pre-flocculation from a Norwegian municipal wastewater treatment plant, Nordre Follo Renseanlegg. The particles in the reactor effluents were characterized with a Malvern Mastersizer and with SF sieves for particle size distribution (PSD). Preliminary jar test trials were performed in order to obtain an optimal dosage of flocculant, mixing and flocculation conditions for subsequent pilot scale testing. The efficiency of two flocculants (cationic polymer based flocculant Superfloc C496 and polyaluminium hydroxide based flocculant, PAX XL-60) was evaluated at pilot scale flocculation. 10 different SF sieve cloth with light opening ranging from 11 μm to 500 μm were tested.

The results indicate that PSDs vary according to the organic loading on the individual reactors with higher organic loading resulting in smaller particle volumes and the particle size peaked around 100 μm in diameter. The results also indicate SF sieves can be used for MBBR biofilm solids separation with and without pre-flocculation. SS and COD removal efficiencies of SF sieves cloths for unflocculated reactor effluent increased with increasing HRT, decreased organic loading and decreasing light opening of the sieves. The formation of a mat on the sieve cloth during filtration was found to lead to reduced SS removal for some sieves and the mat were found to be clogged quickly after formation. Higher hydraulic capacities lead to lower SS removal efficiencies in most cases and the hydraulic capacities decreased with decreasing light opening.

Flocculation changed the particle size characteristics of the reactor effluent and the hydraulic capacities of the sieve cloths. Flocculating with Superfloc C496 shifted PSD towards larger size range and the SS removal efficiency improved for SF sieves in the larger light opening ranges but resulted in reduced hydraulic capacities. Flocculating with PAX XL-60 increased the percentage of smaller particle sizes, lowered overall SS removal efficiencies with negative removal in the larger light opening ranges and lowered hydraulic capacities.

Online characterization of flocculation enabled the flocculation time during pilot scale flocculation studies to be optimised. It was found that with Superfloc C496, the minimum flocculation time for the maximum floc size to be achieved is 6 minutes whereas with PAX XL60, the minimum flocculation time is 9 minutes. Image analysis of the flocs also suggest stirrer design and flocculant have an influence on the shape and structure of the flocs.