Aeration performanceFrom the comparison of the BOD removal rates, it was found that the removal rate was greatly improved: the removal rate of the AQUARATOR was 84.1 to 93.1%,

while the removal rate of the surface aerator was 34.7%. It was confirmed from these results that stable BOD process can be performed by replacing the surfaceaerator with the AQUARATOR. In addition, it was found that mixed liquor dissolved oxygen (MLDO) condition in tanks using the AQUARATOR were improved.

Improving the Palm Oil Mill Effluent by Activated Sludge Process UsingSubmerged Mechanical Aerator/Agitator

YAHAYA HAWARI; IKUMA SONDA; HIROTAKA KAWASHIMA; ZAHARI MOHAMAD

According to the environmental regulation regarding palm oil mill effluent (POME) settled in 1978, it is in progress. Meanwhile in places whereto lower the biological oxygen demand (BOD3) in unprocessed POME, from 25,000 mg liter-1 even lower to BOD 5,000 mg liter-1, and has beenlowered to BOD: 100 mg liter-1 presently. Improvement efforts to reduce this even lower to BOD 50 mg liter-1 are in progress. Meanwhile in places where wastes are discharged into rivers, a progress in research and development (R&D) to achieve the target BOD: 20 mg liter-1 is expected.

To solve these problems, for the purpose of stable improvement in water quality in the aeration tank by the activated sludge process andin particular for the purpose of realization of BOD reduction, we have examined improvement in the aeration tank by replacing the surface aeratorwith the submerged mechanical aerator/agitator which has an “agitation function” and an “agitation diffusing function” separately, intended for usein aeration tanks of polishing plants which adopt the surface aerator.

Introduction

AQUARATOR (Submerged mechanical aerator/agitator)・Installation model: AQUARATOR F-75 (7.5 kW), 3 units (1 unit per tank)・Advantage1. The submerged mechanical aerator/agitator separates the power source for the

two main functions for aeration which are the supplying of air and agitation/aeration. 2. The submerged mechanical aerator/agitator is able to be used flexibly as aerobic

agitation or anaerobic agitation.3. The simple structure leads to no clogging.4. No scattering wastewater/sludge and no occurring noise/vibration.

Change from Surface Aerator to AQUARATORTarget polishing plant

POME volume: 54 m3 hr-1 (1296 m3 day-1)Treatment system: Activated sludge method (ASM) HRT: 1.8 day (43 hr)Number of aeration tank: 3 tanksAeration Tank size (AT): Tank volumes : 771 m3 tank-1 (2313 m3 3 tanks-1)

Upper: Length = 18.5 m Width = 18.5 mLower: Length = 11.3 m Width = 11.3 m Water depth = 3.6 m

Existing aeration device: Surface aerator 15kW (2 units),11kW (1 unit)Agitation performance

The mixed liquor suspended solids (MLSS) and flow velocity were measuredin the respective aeration tanks using the surface aerator and AQUARATOR toinvestigate the actual states of agitation.Aeration performance

The aeration performance was determined by examining water quality throughthe comparison with the ASM process by the surface aerator and AQUARATOR.

Result

HANSHIN ENGINEERING Co., Ltd.2-26-7 Shikanjima, Konohana-ku, Osaka 554-0014, Japan TEL: +81-(0)6-6461-6551 / FAX: +81-(0)6-6461-6555 / Email: info@hanshin-pm.co.jp / Website: http://www.hanshin-pm.co.jp/

Condition of aeration tankIn order to confirm the activated state of the sludge in

the aeration tanks using the AQUARATOR, sludge wasobserved by means of microscopy. Then, it was confirmedthat a protozoa such as Vorticella, Arcella, Trochilia,Euglypha, etc. was existed in flock, and the results showedfavorable conditions.

Herein, since a variety of microorganisms whichappear when the process is successful are present, itwas determined that sufficient degradation of organicmatters was taking place while nitrification was alsoallowed to occur.

AT1Sedimentation tank

AQUARATORRSS: Return suspended solids

Waste activated sludge (Excess sludge)

Influent Effluent AT2 AT3

Figure 3. Change from surface aerator to AQUARATOR at activated sludge method (ASM) for polishing plant

Figure 8. Microscopic image of activated sludge and condition of aeration tankafter installation of AQUARATOR

Figure 1. Project background

Figure 5. Flow velocity for surface aerator in aeration tank Figure 6. Flow velocity for AQUARATOR in aeration tank

0.0

10.0

20.0

30.0

40.0

50.0

0 60 120 180

Aver

age

flow

vel

ocity

(cm

sec

-1)

Measurement time (sec)

Upper: AVG=0.386m/sec (WD = 0.10 m)Middle: AVG=0.162m/sec (WD = 2.0 m)Lower: AVG=0.070m/sec (WD = 3.5 m)

0.0

10.0

20.0

30.0

40.0

50.0

0 60 120 180

Aver

age

flow

vel

ocity

(cm

sec

-1)

Measurement time (sec)

Upper: AVG=0.320m/sec (WD = 0.10 m)Middle: AVG=0.307m/sec (WD = 2.0 m)Lower: AVG=0.296m/sec (WD = 3.5 m)

The flow velocity are almost same between upper and bottom area in pond.= There is no accumulation of sludge. = Mixing condition is very good.

The flow velocity varies between upper and bottom area in pond.= There is accumulation of sludge.= Mixing condition is not so good as it looks.

Surface aerator

AQUARATOR

Agitation performanceThe investigation on the agitation state by determining MLSS and flow velocity showed that the surface aerator provided good agitation state on the surface, but it

reduces the aeration capacity (reaction capacity) because the agitation flow does not sufficiently reach the bottom of the tank, which results in settling of sludge. It isdetermined that the AQUARATOR which realizes a turbulent flow state by underwater agitation is effective in solving this problem, and thatthe agitation state in the aeration tank was greatly improved as a result.

Figure 2. The appearance and feature of AQUARATOR (Submerged mechanical aerator/agitator）

1) High air miniaturization (Miniaturize air bubble, and increase gas-liquid contact area)

2) No clogging

Bottom inflow increases the time air bubbles are contained within the tank

Air from blower

1) Aeration over long distances wide areas with an oblique flow discharge

2) Increased discharge strength through petal- shaped segmentation

Discharge

Inflow

Impeller and Air dispersing rotor

Submerged driving unit

Figure 7. Comparison of the removal rate of the BOD and MLDOconcentration between surface aerator and AQUARATOR

0

10

20

30

40

50

60

70

80

90

100

Surface aeratorInfluent BOD 349mg liter-1

AQUARATORInfluent BOD 331mg liter-1

AQUARATOR Influent BOD 394mg liter-1

AQUARATOR Influent BOD 348mg liter-1

34.7 %

87.9 %93.1 %

84.1 %

BO

D re

duct

ion

ratio

(%)

Improvement of BOD reduction ratio and MLDO

MLDO : 0.3 to 0.98 mg liter-1 (AT1) MLDO : 3.5 to 6.51 mg liter-1 (AT2) MLDO : 6.0 to 7.59 mg liter-1 (AT3)

MLDO : 0.07 to 0.39 mg liter-1 (AT1) MLDO : 0.08 to 0.21 mg liter-1 (AT2) MLDO : 0.13 to 0.39 mg liter-1 (AT3)

Figure 4. Comparison of MLSS condition in aeration tank

1

10

100

1000

10000

100000

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Measurement point for Surface aerator (SA)Measurement point for AQUARATOR (AQ)

MLS

S co

nditi

on (m

g lit

er-1

)

Water depth (m)

UpperAQ = 1450mg/LSA = 1350mg/L

MiddleAQ = 1560mg/LSA = 1630mg/L

LowerAQ = 1460mg/LSA = 40000-44000mg/L

Lack of agitation at bottom of tank