This trial was carried out in the waste water treatment plant of a paper mill with severe bulking problems. Large amounts of suspended solids regularly overflowed from the secondary clarifier and the aerated lagoon had a scum layer of about 20cm on top. This scum layer contained a lot of filamentous bacteria. The effluent from the clarifier flowed into a river resulting in visual floating clumps and foam on the rivers surface.
The predominant filamentous micro-organism present in the waste water treatment plant was Type 021N (based on the identification system of Eikelboom). Chlorination of the recycled sludge, followed by reseeding of the aerated lagoon with sludge from the municipal waste water treatment plant gave positive results for a short period only and this procedure had to be repeated at least four times a year.
Characteristics of the Waste Water Treatment Plant
The aerated lagoon has a volume of 11,000m³and has seven aerators.
Configuration of the lagoon:
- the lagoon is split into two completely mixed parts by a flexible curtain.
- The first part has four aerators and the second part has three.
- Retention time of effluent in the lagoon is 7-8 days.
- Effluent flow into the aerated lagoon = 1500m³/day (average).
- Volume of clarifier = 500m³.
- Diameter of clarifier = 16m.
- A part of sludge (50%) is re-circulated to the lagoon.
- COD of effluent = 255-3500mg/l.
- BOD of effluent = 1500-1700mg/l (maximum of 2000mg/l).
- Suspended Solids Concentration of effluent = 200-300mg/l.
- Temperature in winter = 14-16ºC, in summer = 25ºC.
- PH in the lagoon = 7.8
- Concentration of sludge in the lagoon: 3-3.5g/l
- Dissolved oxygen level in the aerated lagoon: sufficient (1-2mg/l).
- COD/N/P = 100/5/1.
Purpose of Trial
To eliminate the presence and reoccurrence of the filamentous micro-organisms that cause bulking. As a result, better settling of the sludge should occur in the clarifier leading to the COD, BOD and Suspended Solid Concentrations levels of the discharged effluent being within the consent levels.
A lab trial was first conducted to determine the concentration of Amnite K100 needed to solve the bulking problem. Treatment of the Mixed Liquor Suspended Solids (MLSS) from the plant with approximately 10g/m3, gave significant improvement in sludge settlement volume. Lower dose rates gave no significant result. The anti-foam agent was used at a concentration of 2 ppm.
10g of Amnite K100/m³was added to the aerated lagoon together with 2 ppm of the selected anti-foam agent. Foam formation occurred in the aerated lagoon immediately upon addition of Amnite K100. The foam formation however, was controlled, and 10-20 cm of foam was observed only at a few square metres of the lagoon. After one hour, no foam was present at all.
No real changes were seen for the first few days. After the fifth day the bulking problems started to disappear in the first part of the lagoon. Microscopic analysis showed a decrease of filamentous bacteria and the amount of floc forming bacteria became dominant again. The filaments that were still present showed less branches and were much shorter than before. Upon addition of lactophenolblue some cells in the filaments did not stain blue anymore, indicating that these cells had died. In the first part of the lagoon nearly all filamentous scum disappeared at the surface of the lagoon. Small amounts were still present in the dead zones (corners of the lagoons). Three weeks after Amnite K100 treatment had started, no changes were detected in the second part of the lagoon. This is probably due to the physical separation made by the flexible curtain. The second part of the lagoon was not being treated as the product was all consumed in the first part of the lagoon itself.
As Amnite K150 was only added one month after the Amnite K100 treatment, the filamentous bacteria stated to develop again.
Following this study the following recommendations were made to fight the bulking problem:
- Continue to add the maintenance product Amnite K150 (0.125g/m³day).
- A reconfiguration of the lagoon into a plug flow using flexible curtains.
- Move the sludge return line to the same place as the influent