Troubleshooting and Optimisation of a Large Lamella and High Rate BAF Plant for Ocean Discharge

In this paper, a case study is presented on the upgrade of a full-scale primary lamella settling process to chemically enhanced primary treatment (CEPT) for the optimisation of a biological aerated filters (BAF) with a peak flow of 36 MGD for with a population equivalent load of approximately 265,000 inhabitants. As filters tend to accumulate solids in the media, the main limitation of operation is an acceptable head loss, which in turn will determine the allowable media depth. As BAF are limited in their solids holding capacities, higher influent loadings lead to more frequent backwashes. The backwash system to assure the removal of excess biomass adds significant cost and operator constraints.To alleviate these drawbacks, several strategies were demonstrated on a full scale plant to increase the removal performance of a biofilm plants consisting of:- a down flow SAF with a deeper bed of fixed plastic media, and countercurrent aeration, achieving carbonaceous matter removal, followed by- an up-flow BAF with granular expanded clay media and co-current aeration, removing the remaining suspended solids.The following plant optimisation options were identified and carried out:- Co-settling of biosolids was implemented by returning backwash water to the front of the lamella primary settlers, and converting the dedicated backwash lamella to another parallel primary settler. This stabilised the solids load to the lamella settlers and provided additional process redundancy.- Chemical treatment was re-implemented first intermittently in April 2004 and then fully in late August to reduce solids load to the biofilter system.- Regular cleaning of the primary lamella settler was introduced to minimise excess settled sludge being carried over to the biofilters;- The backwash routine and procedures were reviewed and intensified to deal with higher solids in the influent and optimise solids extraction from the biofilters.