WET News

Robust quality control in the field should dovetail through the 'chain of custody' 16 WET NEWS OctObeR 2017 Time to move? F our years ago, we began looking to develop a mod- ular biological treatment system for the UK water market. We had a few basic criteria, the most obvious being that what- ever we came up with needed to offer a significant advantage over the Submerged Aerated Fil- ter (SAF) systems that currently dominate this sector. There was little point in us spending money on our own version of a SAF just to compete with exist- ing products. We also needed to come up with something that offered both a process and financial advantage over the products currently on the mar- ket. So why did our thinking then turn to Moving Bed Bio- Reactors (MBBR) – hasn't the technology been around for a while? Well, while it is true it is not a new technology (having been in successful operation in Scandi- navia since the late 1980s) its use has yet to become estab- lished in the UK. This surprised us as, in comparison to SAF sys- tems, the MBBR process offers greater treatment capacity per unit volume and hence on a like for like basis is significantly less expensive. Although this bene- fit seemed clear we faced a dilemma, should we invest in the in-house development of our own MBBR and run the risk of the technology not being accepted by the largely conserv- ative UK water treatment industry? We decided the benefits were too good to be ignored but to help reduce this risk of rejection by the sector we built two self- contained MBBR units in half- sized (10Œ) shipping containers. These can be rapidly deployed to site to demonstrate the upstream and downstream processes when deploying the technology. This is particularly important for hire installations, where it is tempting to just para- chute the unit in without fully considering its operational con- text. Due to its high treatment capacity, the residence time in the MBBR is relatively short, therefore to get the maximum performance from the unit it is important to feed it at as near a constant rate as possible. Simply feeding it from a wet well, using on-off float control- led pumps, creates surges through the unit, causing fluc- tuations in the applied BOD load. To prevent this, it is usu- ally necessary to either feed the unit using a variable speed pump or install a balance / surge tank immediately upstream of the unit. The primary purpose of the MBBR is the removal of dis- solved organic matter. Unlike conventional biological treat- ment systems, such as activated sludge, the retention time in the MBBR is insufficient to allow all the fine, non-settling organic particles to be either captured or broken down and removed. In addition, when operating at the high loading rates a MBBR can sustain, the biomass growth tends to be very fine, making solid removal by settlement dif- ficult. Therefore, it is necessary to look at alternative methods of solids removal. AŒer all, there is little point in running a MBBR with a hydraulic retention time sufficiently long to allow the breakdown of organic particles, as this negates the operational and cost advantages offered by the MBBR! So instead of just looking at the performance of the MBBR in either coagulant and/or floccu- lation to improve the settleabil- ity of the solids, or DAF to float the solids off. We have also learnt a lot about the use of the MBBR on nitrification projects. The key is ensuring that virtually all the soluble BOD is first removed as otherwise the carbonaceous bacteria will out compete the nitrifiers. Common practice in Scandinavia, for example, is to sub-divide the MBBR into three stages, with the first two used as roughing and polishing stages for BOD removal, thereby allow- ing a healthy population of nitrifying bacteria to become established in Stage III. Consequently, sizing the upfront stages correctly is as crucial as sizing the nitrification stage, as even the short-term carry over of dissolved BOD into Stage III can adversely affect nitrification. During the early stages of developing our MBBR we installed a temporary unit at a works that was subject to weekly short-term spikes in dis- solved BOD from an industrial process. These regularly over- whelmed the BOD removal capacity of Stages I and II (which had been sized based on historic data that did not include the spikes). As a result, the plant never managed to develop a fully effective population of nitrifying bacteria in Stage III. In designing a MBBR instal- lation it is necessary to: Consider how best to feed the unit at a reasonably constant flow rate Assess the performance of the MBBR based on its ability to reduce the dissolved BOD/ COD rather than just its abil- ity to reduce the total BOD/ COD MbbRs have been tested on a number of different applications in both the industrial and municipal sectors two self-contained MbbR units were built in half-sized shipping containers for rapid deployment to site "So instead of just looking at the performance of the MBBR in isolation, it is necessary to look at the MBBR and the downstream solids removal stage as a combined package…" Richard Coulton advantages of the process with- out clients having to commit to a permanent plant. Over the past four years we have had the opportunity to test our MBBRs on a number of dif- ferent applications in both the industrial and municipal sec- tors. These have ranged from use as a roughing filter at the head of the work, through to use as a nitrification filter at the backend of the works. During this time, we have not only dem- onstrated that our initial assumption that MBBRs offer significant process advantages over the existing technology is correct, but we have also gained valuable experience in how to get the best out of this technology. For instance, whilst the biol- ogy used by the MBBR is identi- cal to other fixed film technol- ogy, during our trials we have discovered there are a lot of sub- tleties in terms of both engineer- ing and performance of a MBBR. Subtleties that we have had to learn the hard way as a result of developing our MBBR system from scratch. Things that are obvious with hindsight but were less so at the time! For example, key to the suc- cessful performance of the unit is understanding the air distri- bution, which has the dual function of maintaining the required level of dissolved oxy- gen and keeping the media in motion. Our early trials simply liŒed the media bed and it is only by experimenting with a full-scale unit that we managed to get the media moving correctly. Beyond these practical con- siderations, our trials have also highlighted the importance of taking into account both isolation, it is necessary to look at the MBBR and the downstream solids removal stage as a combined package, with the MBBR removing the dissolved BOD and the down- stream process removing both settleable and non-settling par- ticulate matter. Whilst this approach is not new, the nature of the solids produced in a highly loaded MBBR may require the use of • INSIGHT Sewage & sludge treatment Is the moment right for the municipal water sector to move to MbbR technology? Yes, says Siltbuster Process Solutions' Richard coulton.

• Cover