Conventional Wastewater Treatment System
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If properly treated, household wastewater is a valuable resource for nutrients used in common fertilizers for horticulture, agriculture and forestry. A conventional centralized treatment plant, in which non-renewable nutrients are wasted, will not sustain the nutrition requirements of a growing global population. (Niemczynowicz)

Conventional wastewater treatment relies on large scale plants It is the preferred form of wastewater treatment in developed countries to a large extent because it is a well-known technology in which civil engineers throughout the world are trained. It also minimizes the area required for treatment per capita, which is an important consideration in urban areas where space is at a premium. Conventional treatment also shortens the period that the wastewater effluent remains in retention ponds and so can treat more effluent over a period of time. However, some of the disadvantages are shown at right

Ecological alternatives to wastewater treatment are newer technologies, often unknown and resisted by the engineering and public works communities. Their land requirements are greater than conventional plants and therefore are widely perceived as not applicable in urban areas where land is scarce. However, where feasible, ecological alternatives provide the advantages shown in the box to the right.

Generalized diagram of alternative wastewater treatment. Click to enlarge.

Disadvantages of Conventional Treatment

• Retrofit or replacement of piping is labor and resource intensive.
• No alternatives in the event of a disaster.
• Is not easily adaptable to new technologies or to varying scales.
• Does not promote water conservation.
• Intensive use of chemicals in the treatment process.
• Recycling of nutrients and reuse of water is not optimized.
• Water quality benefits are inconclusive.
• High capital and maintenance cost for plants and supporting infrastructure.

Advantages of Ecological Alternatives

• Reclamation of nutrients
• May be built on any scale
• Flexible and adaptable to a variety of sites
• Are more "transparent"
• Provide useable open space for recreation or wildlife habitat
• Low capital and maintenance costs
• Little or no chemical use
• Decentralized locations
• Educational value

Ecological Wastewater Treatment Options Examined

The study team examined two major system types most closely for applications in Cantarrana: constructed wetlands (both surface and subsurface flow systems) and aquaculture systems. An overview of basic characteristics of each system follows. It is important to bear in mind that alternative wastewater treatment systems are quite flexible, and any system that the community eventually develops may use a combination of components from any of the major system types. In addition, other wastewater treatment strategies, such as source separation of greyawaters, blackwaters, and urban runoff can further increase the efficiency and effectiveness of such a system.

Constructed Wetlands

"Wetlands are land where the water surface is at or above the ground surface for a long enough time each year to maintain saturated soil conditions and the growth of related vegetation."* There are three major types of constructed wetlands that mimic these natural systems, freewater surface (FWS), horizontal subsurface flow (HSF), and vertical flow (VF) systems. These systems consist of a series of connected ponds or reed beds through which the wastewater flows. All constructed wetlands contain an impermeable basin that prevents wastewater from infiltrating and potentially contaminating groundwater. Aquatic plants are introduced within constructed wetland systems that are capable of thriving in saturated, nutrient-rich conditions. Because constructed wetlands mimic natural native systems, there is often a level of increasing native species diversity as the system progresses and nutrient and organic compound levels decrease.

Constructed wetlands can be especially appropriate in developing countries. Their enormous versatility suits them to a wide range of size and function requirements: flow systems can be open (FWS) horizontal (HSF), vertical (VF) or any combination of the three.

Enhancing this flexibility is the variety of emergent rooted and floating aquatic plants that system designers can include. A tropical climate is an especially good environment for constructed wetlands due to the warmer temperatures that increase biological activity and removal efficiency. Other major benefits of constructed wetlands are that they can provide both wildlife habitat and, in the later stages of the process, open or recreational space.

Constructed wetlands also require more land than conventional systems, but the use of land is less intensive. Proper design and maintenance will ensure a successful system. Therefore potential limitations must also be considered. Without contained separation of solids and up-front filtering, odor will at times be significant. Relative to conventional treatment, constructed wetlands have low maintenance requirements. However, managers must remove sediments and harvest plants every 15-20 years.* Harvest and replanting depends on the ability of the system to absorb phosphorous, which depends on the loading rate of the influent. Nutrient recycling with constructed wetlands is among the lowest among ecological treatment systems. Constructed wetlands also require more land than conventional systems, but the use of land is less intensive. Proper design and maintenance will ensure a successful system. Therefore potential limitations must also be considered. Without contained separation of solids and up-front filtering, odor will at times be significant. Relative to conventional treatment, constructed wetlands have low maintenance requirements. However, managers must remove sediments.

Freewater Surface Systems

A Freewater Surface System (FWS) contains open water ponds, using settling as the major treatment mechanism. In addition to settling, flow between wetland plants and percolation into the upper part of the root zone provide a more passive type of filtration than a HSF or VF system. FWS are often densely vegetated with floating or rooted emergent plants that grow at a water depth of 40 centimeters.*

Horizontal Subsurface (HSF) and Vertical Flow (VF) Systems

In a HSF or VF system, wastewater flows below the surface within a porous, planted substrate, using filtering as the primary treatment technique. Filtration systems can be designed in a variety of shapes using a variety of mediums. Substrates suitable for filtration include sand, gravel, LECA, or a mixture. Plants grow in this saturated substrate and the wastewater filters through the root zone. The planted root-zone acts as a biofilter, trapping suspended solids, organic matter, nutrients, and pollutants.

The benefit of a HSF or VF system over a FWS is a shorter retention time and higher removal efficiency. However, often the filtering substrate within a HSF or VF system needs more rapid replacement. For these reasons, phases 1-3 of the recommended system for Cantarrana combines FWS with HSF, in order to exploit the benefits of each system type.

Free Surface Flow Wetland

Conveyance Channel with gravel filtration dam

Artistic constructed wetland near residences

Aquaculture

Aquaculture is one of the oldest techniques used for wastewater treatment. The natural process of an aquatic food chain breaks down organic material. Systems range from indoor and outdoor, containerized mesocosms to large wastewater-fed aquaculture ponds used for fish production in developing countries. Depending on the climatic conditions, systems can contain smaller plant and animal organisms such as microscopic bacteria, zooplankton, and phytoplankton and larger species such as snails, water hyacinths, pistia, carp, tilapia and a variety of other plants and animals.

Usually solids, fats and feces are separated mechanically or through settling. Liquid with suspended solids may go directly into the system, or undergo pre-treatment before entering the constructed food chain. Systems sometimes use anaerobic treatment to further degrade organic compounds and precipitate metal sulfides into ores. Although not necessary, these initial stages and techniques can insure the protection of organisms within the food chain. The use of these up-front techniques depends on the quality of the influent, desired effluent quality and the economic situation of the community.

Aquaculture systems can provide many benefits. Compared to conventional and constructed wetland systems, aquaculture has many opportunities for more efficient nutrient recycling. In addition to the use of effluent for fertilizer, communities may use harvested plants to feed livestock, compost them for fertilizer, or use them in combination with sludge to produce biogas. Today, systems throughout China and India raise fish as a source of protein. * Such systems are also valuable sources for all levels of teaching and research within biological sciences or engineering. In tropical climates, costs are less, as systems are more productive in year-round warm outdoor environments.

Despite the many benefits, aquaculture systems do have some limitations. Generally, aquaculture systems are more expensive than wetlands or filtration systems, and have longer retention times. They are also more delicate, due to the presence of aquatic micro and macro organisms. There is the risk of wastewater contamination by other substances or user negligence, which can make maintenance and operation more of a challenge.

Alternative Systems in Havana

Infrastructure planners and community leaders must consider the appropriateness of applying ecological wastewater treatment techniques to the neighborhoods of Havana within the context of the larger regional development plans. Currently there are plans to increase the operational capacity of the existing María del Carmen treatment plant and to construct one new plant at Puentes Grandes and one in the Luyanó River Basin. The plant on the Bay will only serve parts of the city which already have existing sewerage. The Puentes Grandes facility is still in the design phase and may not be completed for many years, if at all.

In certain areas of the city, where there is sufficient infrastructure and available financing, a central treatment plant may be the best option. However, for the parts of Havana — such as Cantarrana — that have no sewer lines, it is more feasible to develop decentralized systems constructed with minimal outside funding, using local materials, and providing additional benefits to the local population.

An ecological treatment system may be constructed in phases, at lower capital and operating costs than conventional systems. An ecological system can also take advantage of underutilized open space in Cantarrana and convert it to trails, parks, agriculture or wildlife habitat in conjunction with the treatment system. Residents indicated in interviews and in the community meeting that open space was lacking in their neighborhood, and meeting participants prioritized a system that would generate useable open space connected to the community.

In addition to treating wastewater, the proposed plant will allow for production of potentially valuable byproducts such as fertilizer, biogas and water for irrigation. With the potential decommissioning of the two factories in the neighborhood, there may be job shortages within the community which could be mitigated in part with the construction and maintenance of an ecological system, but also through the creation of new employment opportunities that utilize the products generated from effluent.

The river itself lends another argument for decentralization of wastewater treatment. Since the Almendares and its tributaries carry such high quantities of wastewater, there is some speculation that piping all of the waste to a central plant will dewater the riverbed completely. The Ejercito Rebelde reservoir must maintain certain volumes to recharge the aquifer that supplies Havana’s drinking water. If centralized treatment projects were to divert substantial quantities of wastewater from its current course in the Almendares basin, it is unlikely that the dam would be able to increase its discharge rate to replace the volume lost.

Currently, ecological wastewater treatment is relatively unknown in Havana, though several Cuban engineers are investigating the use of constructed wetlands. Dr. Celia Rodríguez of the Center for Hydrologic Studies at the Instituto Superior Politécnico José A. Echevarría (ISPJAE) has identified tropical parameters for the application of the various ecological wastewater treatment systems in Cuba. Dr. Rodríguez and Yociel Marrero have constructed demonstration wetlands at IPSJAE in Havana, and the PMH is in the process of building a constructed wetland system in the neighborhood of Pogolotti, south of Cantarrana. This wetland, when completed, will provide treatment for 500 people at a total cost of around $18,000 U.S. The Canadian International Development Agency (CIDA) contributed $10,000 for construction and local microbrigadas provided the labor. Construction on the wetland has been halted until the remainder of the funding becomes available.

Although these projects have demonstrated the viability of these ecological wastewater options in Cuba, more working examples of these technologies are necessary. Systems in Cantarrana and Pogolotti can educate and convince Cuban decision-makers and technical experts about the their feasibility. Developing these low-cost systems will visibly improve the quality of life in poor neighborhoods such as Cantarrana, and serve as models that can be replicated in other parts of Cuba, as well as in other developing countries.

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