The effect of high salinity on the performances of activated sludge process and plastic trickling filter

by Wong, Yiu-kam

(Uncorrected OCR) Abstract of thesis entitled ' The Effect of High Salinity on the Performances of Activated Sludge Process and Plastic Trickling Filter ?submitted by Wong Yiu

Kam for the degree of Master of Philosophy at the University of Hong Kong in April 19$1 ?

?

ABSTRACT

The employment of sea water as flushing water has been practised in many areas in Hong Kong for quite a long time. As the supply of fresh water is becoming tight due to increase in population and exhaustion of suitable reservoir sites, it is ant icipated that sea water flushing will be practised to a greater extent in the future.

The high salinity associated with sea water will pose threat to the biological treatment methods currently employed for sewage treatment. Flushing water makes up approximately one-third of the total sewage quantity. Typical sea water has a salinlty of 33 000 mg/l and thus the salinity of sewage from sea water flushing areas will be around 11 000 mg/l. This research is carried out to investigate the effect of high salinity on two treatment methods the Activated Sludge Process and the High-rate Plastic Trickling Filter.

Laboratory scale model is used to simulate the Activated Sludge Process. Synthetic waste using peptone as the limiting nutrient is employed as the influent sewage. Five dosages of salt are utilized from 5 000 mg/l to 25 000 mg/l in 5 000 mg/l intervals. The performance of the Activated Sludge Process is

then evaluated in terms of the 5-day Biochemical Oxygen Demand ( BOD5 ) removal.

Experimental results show that when salt is added to the Activated Sludge Process, it shows deterioration in performance, the extent of which depends on the concentration of salt applied. However, the deterioration is transient and the plant will recover in a time

also dependent on the concentration of salt. The same

deterioration in performance is observed when fresh

water sewage is fed to an Activated Sludge Process previously fed with salt water sewage.

On the other hand, the study on the Plastic Trickling Filter is conducted in the Government's Shek Wu Hui Sewage Treatment Works employing their pilotscale Trickling Filter with plastic filter media. Only one salt concentration of 5 000 mg/l is investigated. Performance of the Filter is evaluated in terms of BOD5 and Suspended Solids ( SS ) removals. Experimental

results show that a change in salt concentration

also brings about a transient deterioration in the

Filter performance.

Under continuous'saline conditio~,' however,

although the Plastic Trickling Filter can manage to

show consistent performance after the initial lag period, the treatment efficiency attained is considerably less than that in the pre-salt operation.