UNEP Logo Appropriate Technology for Sewage Pollution Control in the Wider Caribbean Region

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Caribbean Environment Programme Technical Report #40 1998 All CEP Technical Reports

Chapter 5.
Site Visits

Site visits were arranged in November 1997 to sewage treatment facilities thought to be typical of facilities currently operating in the region. Facilities were chosen in three countries: Venezuela, Trinidad, and St. Lucia. Venezuela was taken as representative of several countries in the region—Spanish-speaking, continental, and urbanised with advancing industrialisation. Trinidad is a larger island republic with a continental geographic connection and a history of both Spanish and British political influence and well-established industrialisation. St. Lucia is a smaller island country with a political history of French and British influence whose economy is more agricultural and tourist-centred. Site visits will be discussed by country visited.

Venezuela

Venezuela is a relatively large Spanish-speaking country on the northern coast of the South American continent, the southern shore of the Caribbean Sea. It is largely urbanised. The City of Caracas is one of the largest cities in South America, with a population estimated at 5 million. The metropolitan areas of Maracaibo and Valencia both have populations in excess of 1 million. Sewage collection by gravity is relatively widespread. It has been estimated that approximately 85 percent of the population of the country is served by piped water, 60 percent by sewage collection facilities, but only 3 percent by sewage treatment (Lansdell, 1996). Currently none of the major cities of Caracas, Maracaibo, or Valencia treat domestic sewage. Isolated industrial discharges receive treatment, but residential areas are typically served by gravity sewers leading to outfalls in nearby rivers or streams.

During the site visit, KCM saw four sewage treatment facilities, two currently operating on the Island of Margarita and two under construction in the vicinity of Lake Valencia. Other facilities are currently under construction for Maracaibo. All of these facilities were designed by the firm of Mark Lansdell Associados in Caracas, and Mr. Lansdell served as an escort for the site visits. Upon completion, the new facilities will increase the population served by domestic sewage treatment in Venezuela from 3 to 25 percent.

Valencia Projects—General

The Valencia sewage treatment project is part of a comprehensive water management scheme for one of the largest fresh water bodies in the northern part of South America. The lake level has been extremely variable over the last 200 years of increasing human use of this broad drainage basin (30 km long by 20 km wide). Until 1978, the lake level fell by 260 mm per year, exposing rich agricultural land for human use. In 1978 it was discovered that the heavily polluted Cabriales River was influencing water quality in a major water supply reservoir. At that time the river was diverted into Lake Valencia, causing a rise in lake level and a decrease in lake water quality. The Valencia sewage treatment project will intercept discharges to the river and convey the sewage to the new Mariposa sewage treatment plant.

Valencia.tif (992674 bytes) A raw sewage outfall into the Cabriales River. The Valencia sewage treatment project will intercept sewage from this discharge and many like it and convey it to the new Mariposa treatment plant.
(Click on the picture to the left to view the photograph.)

The Valencia sewage treatment project was conceived to correct water quality and lake level problems. The project includes 90 km of interceptor sewers, 17 km of sewage pumping mains and associated pumping stations, and three major treatment facilities to serve a population of 3.4 million by 2015. The project will treat domestic and industrial wastes, control the level of Lake Valencia, and produce effluent for irrigation and indirect urban uses (Lansdell and Carbonnell, 1991). Project components have been under construction since 1988. To date, major portions of the interceptor and sewage pumping system have been constructed. Work is underway at all three treatment plants. A construction site for a siphon structure under the Cabriales River and two treatment plants, La Mariposa and Los Guayos, were included in the site visit.

The Lake Valencia projects have been funded by a major grant from the Inter American Development Bank. The total cost of the projects, including potable water supply aqueducts and treatment plants and the sanitary sewage facilities, has been reported as $125 million US (Republica de Venezuela, 1993.)

La Mariposa

The Mariposa plant was designed to receive wastewater from a population of 770,000. It will serve the city of Valencia on the western side of Lake Valencia. The plant will provide tertiary-treated effluent with partial nutrient removal for transfer out of the Lake Valencia basin for indirect potable water reuse. The plant includes a simple headworks structure with manual bar racks and grit removal. It features activated sludge treatment using a continuous-level cyclic feed aeration system called a modified sequencing batch reactor (MSBR). This process is further discussed below in the description of the Juangriego treatment plant. The plant has four MSBR treatment modules, each with a volume of 45,000 m3 and a capacity to serve a population of 200,000, or 51,800 m3/day of influent sewage flow. The design hydraulic residence time for each of the MSBR basins is approximately 21 hours.

Mariposa.tif (982002 bytes) MSBR tanks at the Mariposa treatment plant will use high-speed direct-srive propeller aerators suspended from concrete access bridges.
(Click on the picture to the left to view the photograph.)

The MSBR tanks use high speed, direct-drive propeller aerators suspended from three concrete access bridges in each module. Each module will be equipped with twenty-two 75-hp aerators and twelve 3.3-kW mixers. Mixers in the first stages of the influent zones of each module will encourage denitrification. An open baffle between the middle, aerated zone and the anoxic influent zone will allow internal recycling; no internal recycle pumps are provided. Clarified effluent from the MSBR treatment units will be filtered in four declining-rate sand filters to remove residual suspended solids and phosphorus. Effluent from the filters will be discharged to a clearwell for filter backwash. Final effluent will be pumped by axial turbine pumps to areas of groundwater recharge and indirect water reuse. Sludge will be pumped from the MSBR basins to densification basins, with the supernatant returned to the plant headworks. Thickened sludge from the densification basins will be pumped to sludge drying basins for dewatering and stabilisation prior to eventual off-site land application as a soil amendment. When completed, the Mariposa plant will provide the most sophisticated treatment of any in the region, through use of simple, appropriate technology.

LosGuayo.tif (977224 bytes) Construction of concrete liner on the side slopes of the primary treatment cells at the Los Guayos treatment plant.
(Click on the picture to the left to view the photograph.)

The Los Guayos treatment plant is a lagoon system designed for a population of 1.5 million. It will serve the community of Los Guayos and others adjacent to the city of Valencia on the western and northern shore of Lake Valencia. The plant includes two primary treatment cells with relatively deep hopper bottoms. Sludge from the cells will drain by gravity to a sludge drying basin. Effluent from the primary cells will discharge to a series of facultative lagoon cells with a total area of approximately 120 hectares. Based on the design influent flow of 2,000 litres per second, or 173,000 m3/day, and an assumed influent concentration of 200 mg/L of BOD, the overall loading on the lagoon system will be approximately 288 kilograms of BOD per day per hectare. The plant features a recirculation canal and submersible pumps to mix treated effluent with influent to the anaerobic cells. Effluent from the plant will be made available for irrigation or discharged to the Los Guayos River, the receiving stream for the current raw sewage discharges of the upstream drainage area.

Dos Cerritos

The Dos Cerritos treatment plant for the city of Porlamar on the Island of Margarita was the first activated sludge plant of any substantial size to be constructed for treatment of domestic sewage in Venezuela. The Island of Margarita is the main coastal tourist attraction in Venezuela. In the early 1970s increases in sewage flows to raw sewage outfalls resulted in increases in faecal coliform concentrations at beaches. This caused significant concern and threat to the tourist economy and a desire for corrective action. The Dos Cerritos plant was first conceived in a master plan prepared for the city of Porlamar in 1975. It was designed in 1980. Project implementation suffered from delays in construction financing, and the plant did not come into service until 1989.

DosCerit.tif (979262 bytes) Aeration tanks at the Dos Cerritos treatment plant.
(Click on the picture to the left to view the photograph.)

The plant provides secondary treatment by the activated sludge process. Effluent from the plant is used for irrigation or discharged to the Caribbean Sea. The plant was designed for a wastewater loading from a population of 200,000. The design influent flow was 600 L/sec or 51,840 m3/day. At the time of the site visit, average plant flow was 31,146 m3/day. Headworks facilities for the plant include manual bar racks and a grit removal channel. The plant has five aeration tanks with a total volume of 17,300 m3 for a total hydraulic residence time of 8 hours under design flow.

Activated sludge is separated in six settling tanks, each 20 meters square for an average overflow rate at the design flow of 0.9 m/hr. The settling tanks have no rake mechanisms. Sludge is withdrawn continuously from four inverted pyramidal pockets through telescoping valves. The combined return activated sludge flow from all six settling tanks is pumped by Archimedes screw pumps into the plant headworks. Effluent from the activated sludge process is discharged to two maturation ponds for natural ultraviolet disinfection. The maturation ponds are 1.5 m deep and designed for an average detention time of five days. At the time of the site visit, the maturation ponds were reducing faecal coliform concentrations in the activated sludge effluent from 270,000 organisms per 100 mL to less than 200 per 100 mL prior to final discharge, through the effects of solar radiation and bacterial die-off in the shallow maturation ponds and without use of chemical disinfectants. The effluent concentration averaged approximately 15 mg/L BOD and 10 mg/L total suspended solids (TSS). The plant was fully nitrifying with effluent ammonia concentrations averaging 0.1 mg/L. Mixed liquor from the activated sludge process is wasted to sludge lagoons, which serve to concentrate and stabilise sludge prior to land application.

Juangriego

The site visit included the treatment plant for the community of Juangriego on the northern coast of the Island of Margarita. This plant currently protects the Bay of Juangriego from sewage contamination that had closed beaches to recreational use by the late 1970s. The plant came on line in 1990. It was designed for a population of 50,000 and an average flow of 10,000 m3/day. Current flows are approximately one-fourth of the design flow. The plant contains the same basic elements as the Dos Cerritos plant—headworks, activated sludge treatment, and maturation ponds for final disinfection. The plant’s activated sludge process is the first operating MSBR system and has served as a model for design of subsequent facilities by the Lansdell firm in Venezuela, including the Mariposa plant in Valencia, and similar facilities for Colonia Tovar, Cruz del Pastel, Cumana East, and Punta Gorda.

JuanGr1.tif (987052 bytes) Simple pneumatically operated gates cyclically feed different zones of the MSBR at the Juangriego treatment plant.
(Click on the picture to the left to view the photograph.)

The MSBR process is an activated sludge process in which different zones in a single activated sludge basin serve alternately for aeration and for sedimentation thus eliminating the need for separate settling tanks and return activated sludge pumping systems. Simple pneumatically operated gates cyclically feed different zones of the MSBR. Unlike conventional sequencing batch reactor (SBR) systems, the MSBR process has a constant water level in the basin and does not result in a significant loss of head. Neither does it produce elevated flow rates from batch discharge of effluent. SBR systems lose up to 3 meters of head across the process and produce a batch discharge up to eight times the average flow into the system. The batch discharge feature of a conventional SBR system requires downstream elements such as filtration, disinfection, and effluent outfalls to be sized for elevated flows. The MSBR process does not share these drawbacks. It has the added advantage that basins can be constructed as excavated earthen basins with a thin concrete or other liner to prevent leakage and erosion. This permits less expensive construction than the concrete-walled basins designed for hydrostatic loads that are typically used for activated sludge aeration.

JuanGr2.tif (968200 bytes) Effluent zone of the Juangriego basin operating in a decant mode.
(Click on the picture to the left to view the photograph.)

The Juangriego plant is divided into nine individual cells through which water flows in a serpentine path. The flow path is reversed on a regular cycle. The three outer cells on each side of the basin are alternately used for sewage feed and aeration and for sedimentation and decanting. The middle three cells in the basin are always used for aeration. The Juangriego plant uses nine floating surface aerators that are alternatively turned on and off in the inlet/sedimentation zones of the basin. The plant has consistently produced effluent in the range of 5 to 15 mg/L of TSS and BOD with less than 200 total coliform organisms per 100 mL. The Juangriego plant was constructed in 1989 for a total cost of $900,000 US. The Dos Cerritos and Juangriego plants are operated by a private contractor, Ejecuciones Terepaima, S.A.

Cost of Sewage Treatment Facilities in Venezuela

The construction cost of completed MSBR activated sludge systems in Venezuela has been in the vicinity of $80 US per m3/day average flow capacity or $20 per population equivalent (Lansdell, M 1996). Comparable costs for construction of activated sludge treatment systems in the United States would be at least 15 times as great. This lower cost is due to many factors. A key factor is lower labour rates, which are a primary element in treatment plant construction costs. Higher temperatures and solar input in Venezuela result in higher growth rates for treatment organisms, allowing relatively small tank volumes for treatment reactors. Use of sludge lagoons for solids stabilisation and drying reduces the cost of the treatment facilities significantly in Venezuela compared to construction in Europe or the United States. In the United States, as much as 50 percent of the capital cost of sewage treatment construction can be for sludge treatment facilities. In Venezuela this ratio can be much less if solar sludge drying in open, unlined earthen basins is used. The experienced cost for construction of lagoon systems in Venezuela is only $4 US per population equivalent, one fifth the cost of activated sludge systems (Lansdell, M 1996). This cost does not include the cost of land.

Lansdell estimates the cost of operation and maintenance of MSBR systems in Venezuela at $2 per person per year, about half of which is for electrical power. This produces a total capital and operating cost for sewage treatment of approximately $5 per capita per annum, about 10 times the cost of lagoon systems in Venezuela, but only 10 percent of the cost of such systems in developed countries in northern climates. The ratio of the cost of sewage treatment to gross national product in Venezuela is estimated at approximately 0.25 percent for activated sludge systems and 0.025 percent for lagoons.

Trinidad

Trinidad and Tobago are the southernmost islands in the Caribbean region. Trinidad is the largest and most heavily populated island in the Eastern Caribbean. It was sighted in 1498 by Christopher Columbus who christened it La Isla de la Trinidad, for the Holy Trinity. The first Spanish settlement was in 1592 and it remained a part of the Spanish Empire until 1797 when it came under British control. It remained dominated by England until its independence as part of the Republic of Trinidad and Tobago in 1962. The island has a mix of urban development, rain-forested mountains and small farming communities. It is relatively heavily industrialised with major petrochemical complexes in the south of the island. Other industries include production of processed foods, fertilisers, cement, steel, and electronics.

The site visit included four sewage treatment facilities on Trinidad, three of them larger facilities serving major urban areas around Port of Spain and San Fernando, the island’s largest urban centres. These plants were operated by the Water and Sewage Authority of the Republic of Trinidad and Tobago. The visit also included a smaller extended aeration treatment plant built for a housing development by the Housing Authority. This plant is typical of the more than 150 package plants treating wastewater from small residential developments on the island.

Beetham Lagoons

The Beetham Treatment Works was designed in 1959 as an oxidation pond or lagoon system to treat sewage from a population of 150,000 in the City of Port of Spain, Trinidad’s largest city. The facility came on line in 1965. Influent sewage arrives at an influent screening works and pumping station just south of the Beetham Highway, from which it is lifted to an above-grade, 48-inch diameter pumping main designed to carry it to the oxidation pond system located in the Lavantile Swamp. The works was designed to discharge into the Caroni River basin, which is a prominent roosting area for the national bird of Trinidad and Tobago, the scarlet ibis. The oxidation ponds include four anaerobic ponds, each 540 feet (165 m) by 520 feet (158 m) in footprint area, and two aerobic ponds, each 1,880 feet (573 m) by 1,120 feet (341 m) in plan dimension. The total oxidation pond area is 122 acres, or 49.5 hectares.

At the time of the site visit, it appeared that much of the lagoon system was filled with accumulations of grit and sludge. The entire surface of the first-stage cells and one of the second stage cells was covered with a dense accumulation of rooted vegetation, including good-sized shrubs. Ponded water was present on only a small part of one of the large second-stage cells, and that may be an accumulation of rain water. At the time of the site visit, little or no sewage appeared to be reaching the lagoon system; instead, it discharged through numerous openings in the 48-inch pumping main into the adjacent mangrove swamp.

Beetham.TIF (954494 bytes) Lagoons at the Beetham Treatment Works are largely covered over with vegetation.
(Click on the picture to the left to view the photograph.)

Influent sewage is a combination of domestic wastewater and rum distillery waste. The sewage is distinctly red and hydrogen sulphide gas is released wherever it is exposed to the atmosphere, in the influent screen works and pump station wet well and at the discharge points in the swamp. Current influent flow rates are unknown because the influent flow meter is not functioning. Phelps and Griffith (1974) attempted to estimate the influent load. They reported that the distillery waste flow was 40,000 gallons per day (151 m3/day) with a BOD strength averaging 25,000 mg/L. This is a BOD loading of 3,791 kg/day. The design BOD loading to the ponds is reported by Phelps and Griffith to have been 15 mgd (57,000 m3/day) at 170 mg/L or 21,267 pounds per day (9,667 kg/day) of BOD. Thus the distillery waste load, which was not taken into account in the design, represents approximately 40 percent of the original design capacity.

Phelps and Griffith estimated the actual combined loading in 1972 to be 12 mgd (45,420 m3/day) at a waste strength of 250 mg/L, or a combined loading of 25,020 pounds per day (11,373 kg/day). At this rate the unit organic loading on the overall lagoon system would be 205 pounds BOD/day/acre or 229 kg BOD/day/hectare. Considering that the current population of Port of Spain is reported at 300,000 and that most of the city is connected to the sewer system, actual loading could be greatly in excess of this, once the planned desludging project is completed to permit reuse of the lagoons for sewage treatment.

Arima.TIF (992182 bytes) Effluent from the secondary sedimentation tanks at the Arima sewage treatment works.
(Click on the picture to the left to view the photograph.)

The Arima sewage treatment works is a trickling filter plant serving an upland area that is an extension of the urbanised area of Port of Spain. The plant was constructed in the early 1960s. It includes a headworks structure with manual bar screens and conventional dry well/wet well influent pumps, an influent flow meter (non-functional), two primary sedimentation tanks, two trickling filter tanks, and two secondary sedimentation tanks. Effluent is discharged to an adjacent river, which appeared to have relatively good water quality at the time of the site visit. Sludge from the primary and secondary sedimentation tanks is pumped to two anaerobic digesters. No instrumentation at the works is in working order, but the main pumping and process equipment has been well maintained by a concerned and competent staff. This plant was the only one visited in Trinidad from which treated effluent was being discharged at the time of the visit. Design data for the plant were not available. With two primary sedimentation tanks estimated at 50 feet (15 m) in diameter at a design average day overflow rate of
1,200 gallons per day per square foot (2.0 m/hr) the equivalent plant capacity would be approximately 5 mgd or 18,000 m
3/day.

San Fernando

The San Fernando sewage treatment works serves industry and residential users of the city of San Fernando, Trinidad’s second largest city, with a population estimated by the operator of the works at 75,000. The works is identical in configuration to the Arima works, including bar screens, influent pumps, primary sedimentation tanks, trickling filters, secondary sedimentation tanks, and anaerobic digesters. In addition, the San Fernando works has a septage receiving station and an aerated grit chamber. At the time of the site visit, the influent wet well and bar screen area were flooded and the influent pumps were off. According to an operator at the works, the pumps were operated 10 to 12 hours per day. The operator indicated that the capacity of the works was 12 mgd, or approximately 50,000 m3/day. The influent flow meter was not functional. The San Fernando works dates from the same period of construction as the Arima and Beetham works, the late 1950s. The receiving water for discharge from the works is an estuarine river adjacent to the plant site. Water quality in the river was evidently quite bad. Its colour was nearly black and its visibility poor.

SanFrndo.TIF (1009852 bytes) The San Fernando sewage treatment works.
(Click on the picture to the left to view the photograph.)

Charlieville

The last plant visited in Trinidad was a small extended aeration plant designed for a residential development in a community called Charlieville, midway between Port of Spain and San Fernando on the west side of the island. Plant construction was completed recently. The plant includes self-priming influent pumps, two elevated concrete aeration tanks with coarse bubble aeration from positive displacement blowers, two rectangular sedimentation tanks, and a chlorine contact channel. Chlorine is delivered in solution from a 150-pound gas/liquid cylinder feed system. A sludge holding tank is included for partial stabilisation of waste sludge prior to discharge to sludge drying beds on-site. Effluent discharge is to a concrete channel leading to a small stream adjacent to the plant site. The plant appeared to be adequately designed, but operators had yet to be fully informed about the nature of its unit processes. The aeration blowers were on at the time of the site visit. The operators indicated that they turned them on and off using a timer. The plant was receiving no sewage at the time of the visit, on a Friday morning. The operators indicated that few houses had been connected to the collection system feeding the new plant. Design data for the plant were not provided. Based on the approximately 1-by-3-m footprint of each of the two rectangular sedimentation tanks at a design overflow rate of 1 m/hr, the capacity of the plant would be in the neighbourhood of 500 m3/day.

ChrlyVil.TIF (993684 bytes) Aeration tank at the small extended aeration plant in Charleyville.
(Click on the picture to the left to view the photograph.)

St. Lucia

The island of St. Lucia is in the southern half of the Eastern Caribbean island chain. It is a teardrop-shaped island roughly 43 kilometres long and 23 kilometres wide. Its interior is very mountainous; the highest point is Mt. Gimie, with a peak elevation of 950 metres. The average temperature is 22 to 27 Celsius, and annual rainfall ranges from 150 to 345 centimetres.

St. Lucia is an independent state within the British Commonwealth with a population of 157,000. One-third of the population lives in Castries; the rest are distributed in small communities and fishing villages throughout the island. Almost all the settlements are within 8 km of the sea, and those that are not are located on streams that flow to the sea. The main industry in St. Lucia is agriculture. The primary exported products are bananas, coconuts, and cocoa. Tourism, another important industry, has been growing quickly in recent years. There has been a surge in new hotel and resort development, particularly on the western coast. Many of the hotels lie in the coastal areas, but others are being developed in the interior.

St. Lucia is served by a collection system only in Castries and parts of Gros Islet. Sewage from Gros Islet is treated in a series of lagoons before being discharged to the sea. Untreated sewage from Castries is discharged directly to the sea through a nearshore outfall. The rest of the population is served by package wastewater treatment plants, septic tanks, outhouses, and other local methods of wastewater disposal.

The site visit included the Rodney Bay Sewage Treatment Works in Gros Islet and four hotel package plants.

RodnyBay.tif (986048 bytes) Lagoon at the Rodney Bay Sewage Treatment Works in Gros Islet.
(Click on the picture to the left to view the photograph.)

Excluding small package plants, the Rodney Bay Sewage Treatment Works is the only sewage treatment facility in St. Lucia. The treatment works is operated by the Water and Sewerage Authority (WASA) and serves a portion of the Gros Islet population in the north of the island. Gros Islet is one of the few areas on the island with large, open spaces. The Rodney Bay facility is an Advanced Integrated Pond System (AIPS) designed by the engineering firm founded by the man that invented the concept, William J. Oswald. It was constructed though a grant from the French govenment. The sewage flows through a screen before entering the first of a series of four lagoons. The first two lagoons are equipped with mechanical surface aerators. Lagoon effluent is discharged to a stream that flows to Rodney Bay. The effluent quality appeared to be good at the time of the site visit. Effluent quality measurements indicate that the colour and dissolved oxygen (DO) level of the effluent vary on daily cycles, suggesting the presence of algae. The lagoons are currently underloaded. WASA anticipates more sewer connections in the future. Design data are presented in Table 5-1. Effluent data are presented in Table 5-2.

 

TABLE 5-1.
DESIGN DATA. RODNEY BAY AIPS.

Parameter

Value

Units

Flow
Design
Current


2,600
800

m3/day

Volume
Pond 1 and 3A
Pond 2 and 3B
Pond 4
Pond 5
Pond 6


25,000
25,000
5,000
7,000
8,000

m3

Depth
Pond 1 / 3A
Pond 2 / 3B
Pond 4
Pond 5
Pond 6


4.5/2.7
4.5/2.7
1.5
1.9
2.7

m

 

 

 

TABLE 5-2.
EFFLUENT QUALITY DATA. RODNEY BAY AIPS. 1996-1997.

Parameter

Oct

Nov

Dec

Jan

Feb

Mar

Apr

May

BOD, mg/L

24

16

21

9

11

15

11

12

TSS, mg/L

18

26

46

41

38

38

39

52

Phosphate, mg/L

4.5

13.1

13.8

10.9

12.6

16.3

11.6

14

Nitrate, mg/L

3.4

1.6

7.5

2.4

2.2

2.5

21.2

1.6

Faecal Coliform, Colonies/100 mL

43

2

23

40

24

2

8

32

pH

7.9

8.3

9.2

8

8.5

9.0

9.5

9.5

 

 Hotel Package Plants

Several large hotels on the island represent a population density too high for septic tanks to be efficient or economical. To avoid pollution of nearby bathing beaches, many of these hotels use small, extended-aeration package plants. The site visit included three extended aeration package plants and one wetland treatment system.

Of the hotel package plants visited, the highest quality effluent was noted at a wetland treatment system for a medium-sized hotel. The treatment process includes pre-treatment with screening and settling. The wastewater then flows into a three-tiered, free-water-surface wetland system dug into a hill. The wetland effluent passes through a filter and then is disinfected with an ultraviolet lamp. Monitoring data showed that the effluent BOD and suspended solids concentrations were typically less than 10 mg/L.

StLucia2.tif (1000732 bytes) Constructed wetlands in a package treatment plant serving a hotel in St. Lucia.
(Click on the picture to the left to view the photograph.)

The extended aeration package plants visited are equipped with most of the processes that an ordinary activated sludge treatment facility uses, but on a smaller scale. All used a process sequence of screening, followed by an aeration basin, followed by sedimentation tanks, followed by chlorine disinfection before discharge or reuse for landscaping. At one of the hotels, an equalisation basin was provided before the aeration basin. Sludge from the sedimentation tanks was recycled back into the aeration basins, with the excess typically wasted to a thickening tank. No design data were obtained for these plants, but it is estimated that most were designed with capacity to serve a population of 500 to 2,000. Overall effluent quality was adequate to excellent, except at one package plant that was not operating adequately due to broken down equipment awaiting replacement.

 

 

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