323

6 Wastewater

6.1 Overview Umgeni Water currently owns and operates the Darvill and Ixopo Wastewater Works (WWWs). It also operates the Howick WWW for uMgungundlovu District Municipality under a management contract, and the Albert Falls North and South WWWs as part of the management of Albert Falls Dam. All existing operations are based on current technology, namely aeration basins for biological nutrient removal and clarifiers for the separation process. New technology such as membrane separation would allow for the treatment of at least a fifty per cent increase on present volumes in activated sludge (i.e. all these plants). Umgeni Water is currently investigating this at pilot scale. The quality of water produced may make newer technology a necessity as DWA raises standards to compensate for increased water demand and population density.

6.2 Darvill Wastewater Works The Darvill WWW (Figure 6.1 and Figure 6.2) serves the Msunduzi Municipality. This WWW has a current biological treatment capacity of 65 Mℓ/day. Current average daily inflow (Nov 2007 to Dec 2012) is approximately 81 Mℓ/day (Figure 6.3) which is more than the plant’s capacity. The graph also indicates the spike in flow in the summer seasons when there is ingress of stormwater into the sewer system.

Figure 6.1 Darvill Wastewater Works.

324

Figure 6.2 General layout of the Darvill WWW.

325

0

20

40

60

80

100

120

140

Jan

-02

No

v-0

2

Sep

-03

Jul-

04

May

-05

Mar

-06

Jan

-07

No

v-0

7

Sep

-08

Jul-

09

May

-10

Mar

-11

Jan

-12

No

v-1

2Ave

rage

Dai

ly ln

flo

w (

Mℓ

/day

)

Umgeni Water is currently investigating options to upgrade Darvill WWW to cope with the increased inflows and predicted wastewater demands. The average dry weather flows (ADWF) within the Darvill WWW catchment are expected to grow to about 90 Mℓ/day by 2020 (Figure 6.4). Hence, it is proposed that the plant be initially upgraded by at least 35 Mℓ/day. The upgrade design will make allowance for expansion in the future to a maximum capacity of 120 Mℓ/day, which is forecast to occur in 2024. As a summary the proposed civil and process infrastructure upgrades at Darvill are as follows:-

Biological treatment

o Convert the existing Aerobic Reactor to an Anaerobic / Anoxic reactor

o Construct a new reinforced concrete Aerobic Reactor (40 150 m3)

Air for Biological Treatment

o Blower House

o 4 x 645 kW Blowers each with a rated delivery of 7 m3/sec @ 90 kPa

o Air Header Mains

o 22 680 diffusers in three aeration lanes

Secondary Settling

o 2 x 35 m diameter secondary settling tanks

Advanced Treatment

o Reinforced Concrete Submerged Membrane Tank

o Submerged Ultra-Filtration (UF) Membrane System complete with suction pumps,

backwash blowers and CIP chemical storage and dosing

The UF Membrane System will be designed, but not constructed until feasibility studies on water reuse at Darvill are complete. The UF Membrane System will produce water of sufficient quality that it can be used as the feed to a water reuse plant. If the water reuse plant does not materialise then water of this quality will not be required and Darvill WWWs effluent can be discharged into the Msunduzi River as normal. More information is available on the Darvill Reuse project in Section 4.

Figure 6.3 Average daily inflow (Mℓ/day) to Darvill WWW.

326

The present method of disposal of sludge by spray irrigation to land is operating adequately, but has its limitations especially as the capacity of the works increases to cope with growth. Alternative methods of sludge treatment and disposal are thus being investigated in order to determine the optimal future disposal strategy. A feasibility report which looks at the management and disposal of sludge produced by the works has been completed and the following recommendations have been included in the proposed upgrade:-

Refurbish the existing Gravity Thickeners

Construct a new sludge treatment building with mechanical thickening of waste activated sludge and digester sludge dewatering plant.

Construct a new sludge sump for blending thickened primary and waste activated sludge and new digester feed pump station

Construct two new “Egg-Shaped” digesters.

Develop a composting facility.

Figure 6.4 Projected Inflow into Darvill WWW

50.00

60.00

70.00

80.00

90.00

100.00

110.00

120.00

130.00

20

03

20

05

20

07

20

09

20

11

20

13

20

15

20

17

20

19

20

21

20

23

20

25

20

27

20

29

INFL

UEN

T V

OLU

ME

(M

ℓ/d

)

YEAR

ADWF

Plant Capacity (Existing)

Plant Capacity (Proposed)

Projected based on Historical growth

327

6.3 Ixopo Wastewater Works Ixopo WWW (Figure 6.5 and Figure 6.6) serves the town of Ixopo in the Sisonke District Municipality. The WWW has a design capacity of 1.0 Mℓ/day and is designed to be doubled. Sludge is dried on beds and disposed of on a local farm that was owned by Umgeni Water, but has been sold to the Sisonke District Municipality. The average daily inflow to the Ixopo WWW is shown in Figure 6.7

Figure 6.5 Ixopo Wastewater Works.

328

Figure 6.6 General layout of the Ixopo WWW.

329

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

Mar

-02

Au

g-0

2

Jan

-03

Jun

-03

No

v-0

3

Ap

r-0

4

Sep

-04

Feb

-05

Jul-

05

De

c-0

5

May

-06

Oct

-06

Mar

-07

Au

g-0

7

Jan

-08

Jun

-08

No

v-0

8

Ap

r-0

9

Sep

-09

Feb

-10

Jul-

10

De

c-1

0

May

-11

Oct

-11

Mar

-12

Au

g-1

2

Ave

rage

Dai

ly I

nfl

ow

s (M

ℓ/d

ay)

Month

An upgrade to the WWW was not envisaged in the near future, but since the implementation of maintenance work in the reticulation in August 2010, the inflow to the works has climbed to 20% above the design capacity. The capacity of the works is being increased with the construction of an additional clarifier (3.25 Mℓ/day), which will take place later this year.

6.4 Howick Wastewater Works and Mpophomeni Wastewater Works

Umgungundlovu District Municipality provides wastewater treatment services for the uMngeni Local Municipality at the Howick WWW. A large pump station at the Mpophomeni WWW site and a set of six of smaller pump stations in Howick transfer wastewater for treatment at Howick WWW. Umgeni Water has a management contract with Umgungundlovu District Municipality to operate this WWW on their behalf. Howick WWW (Figure 6.8) has a design capacity of 6.8 Mℓ/day and is currently treating 5 Mℓ/day. The WWW site is currently fully utilised. Mechanical dewatering equipment has been ordered. If additional treatment capacity is required, the WWW will have to be upgraded to a membrane bioreactor (MBR) or similar.

Figure 6.7 Average daily inflow (Mℓ/day) Ixopo WWW

330

Mpophomeni WWW is currently not functioning, and is only used as a transfer station to Howick WWW. This WWW will be reinstated as a treatment facility as soon as the necessary finances have been raised by Umgungundlovu District Municipality. The proposed plant will be designed to treat 6 Mℓ/day. The site has space for at least 20 Mℓ/day. The area served by the Howick WWW and Mpophomeni WWW is in a high rate of expansion. Despite this, there appears to be relatively little forward planning of the wastewater infrastructure being carried out. The combination of this growth combined with the restrictions on the size of the Howick WWW site could limit development. This situation needs to be rectified.

6.5 Albert Falls North and South Wastewater Works

The two Albert Falls WWWs (Figure 6.9) are transportable activated sludge units installed for the construction of the dam in the early 1970s. They were probably originally designed for approximately 100 m3/day (0.1 Mℓ/day). The bulk of the current feed to these WWWs is from conservancy tank contents delivered by tanker from the Albert Falls and Bon Accord resorts. Umgeni Water refurbished the WWW to a Sequencing Batch Reactor (SBR) configuration with a treatment capacity of 40 m3/day to suit present demands.

Figure 6.8 Howick Wastewater Works.

Figure 6.9 Albert Falls Wastewater Works.