After setting our goal to decompose synthetic dye with laccases, we seek to find out more information of biological sewage treatment and practical problems that required to be addressed. We also want to find out how our project can be applied to real world sewage treatment, and what change should we make to improve our project.

Therefore, we visited Hanghua sewage plant（World Trade Center, Beijing, China). Hanghua sewage plant is experienced in using biology method to decompose pollutant while they also had used other ways, including chemical and physical methods. Mr. Li, the manager of the sewage plant, told us lots of information we needed.

During the interview, we found out the situation of each current methods.

First of all, we learned that their previous chemical and physical techniques such as active carbon adsorption and oxidation method, being expensive in cost for both methods, the sewage plant had to switched to biological treatment.

Hanghua sewage plant mainly uses integrated sewage treatment device, in which a critical step is to add bacteria to decompose pollutant. Specially engineered bacteria are able to reproduce rapidly in sewage water, and this allows the factory to process waste water with relatively lower financial requirement.

However, the bacteria used in sewage treatment may release toxic gases such H2S. The plant has to turn on the ventilation system periodically, which consumes large amount of electric power and produce loud noise. As time goes by, the advantage of lower cost is offset by electricity bills, and the higher payment for their workers working unfriendly work environment.

In traditional biological treatment method, bacteria are usually injected into waste water directly, floating, growing, as well as continuously being removed by the outflow. The dynamic balance of bacteria concentration might be broken by increasing the total flow of sewage water. Under extreme circumstances, growth rate of bacteria might be overwhelmed by the loss rate through outflow. Moreover, workers in the sewage treatment plant told us, due to wear and tear since 2003, and other undiscovered problems, the total ability of sewage treatment had dropped 40%. To address this issue, we need to develop a sustainable consumptive material.

By discover shortcoming of traditional treatment method, we found some critical factors that are worth considering in our project.

The first factor is the low concentration of bacteria/enzyme during treatment. A high flow rate may wash away bacteria fast, lowering the concentration of bacteria/enzymes as well as the chance and duration of contact between pollutant and enzymes. This remind us to adjust our project in two aspects:
(1) Attach bacteria/Laccase to some fixed matrix, preventing them from being washed away by water flow.
(2) Use lots of small plastic beads to expand surface area, increasing the chance of contact between pollutant and enzymes.

In combination of the two aspects, we may achieve a higher concentration of laccase and longer duration of contact between pollutant and enzymes. Moreover, because the laccase processes the sewage without releasing toxic gas, it eliminates the unnecessary cost of using extraction pump and extra employee payment, which as a result enables factory to utilize the biggest advantage of biology method, inexpensive price.

The second factor to consider is the competition among bacteria species. Through human practice, we realized that the composition of sewage disposal is very complicated. There are usually more than one species of microorganism competing in the same environment, which may negatively affect the growth of target bacteria. By allowing our bacteria to produce biofilm, they could get an advantage of growth by sharing nutrients and getting sheltered from competitors.

Finally, we came up with the idea, using PHA plastic beads coated with biofilm and biofilm producing bacteria, then display laccase enzyme on biofilm by adding SpyTag to Biofilm and SpyCatcher to CotA laccase. PHA can also act as a carbon source to bacteria, maintaining growth of bacteria and production of biofilm for longer period. This may further reduce cost by replacing biomaterials less frequently.

In conclusion, through the biology method, we found that the expensive cost and decreasing effectiveness are two main problems for traditional biological treatment method. These experiences from human practice has inspired us with the idea of designing our Biofilm × Laccase system. In the near future, we will build a filter with PHA beads coated with laccase linked biofilm and optimize the filter for real world sewage treatment.