TLTG "Kolam Stabilisasi"

LOW-COST WATER SUPPLY AND SANITATION

“Waste-Stabilization Ponds”

Introduction

Sewage from waterbone sanitation systems must be treated before it is disposed of or reused, because it contains wastes which pollute the environment. This treatment has two objectives:

·         To remove organic matter from the sewage which causes pollution;

·         To remove pathogens (disease-causing organism) which cause serious health risks.

Sewage treatment processes in developing countries should;

·         Be effective, low-cost and simple to construct and operate;

·         Need little imported equipment.

Advantages and Disadvantages

Waste-stabilization pond systems are simple to build, reliable, and easy to maintain. Stabilization ponds are a low-cost process for treating sewage. They require little or no imported mechanical equipment. Pond systems provide pathogen removal, which is better than convensional treatment processes. By keeping raw sewage in a series of shallow ponds for two to three weeks, safe levels of BOD and pathogen removal can be achieved. The ponds give a consistent standard of treatment and are not sensitive to sudden load increases.

The only disadvantage is that waste-stabilization ponds require a substantial amount of land.

Anaerobic Ponds

Anaerobic ponds are used to settle out and break down organic matter

They are always placed first in a series of  waste-stabilization ponds. Two processes take place in anaerobic ponds:

·         Solid in the incoming sewage settle to the bottom of the pond and form a layer of sludge.

·         Anaerobic bacteria (bacteria that require no oxygen) break down the organic matter in the sludge. This breakdown produces gas (which is lost in the atmosphere) and some soluble products which pass into other ponds. This process causes a very low build-up of sludge in the anaerobic pond.

Facultative Ponds

Facultative ponds are used BOD and pathogen removal.

They can be the first pond in a series of treatment ponds or they can receive effluent from an aerobic pond. In this pond:

·         Some of the suspended solids settle to the bottom where they are digested anaerobically. This bottom layer is called the anaerobic layer. Thirty percent of the BOD reduction of the pond occurs in this layer.

·          There is a layer above the anaerobic layer in which oxygen is present. Algae growing  in this layer produce the oxygen by photosynthesis . The algae obtain the nutrients they require to survive from by-products of aerobic bacteria, which are also present in this layer. Aerobic bacteria require oxygen to survive and the bacteria get this in turn from algae (as a by-product). The interdependence between the algae and aerobic bacteria is called symbiosis (figure 2).

Maturation Ponds

Maturation ponds are used to upgrade the effluent from a facultative pond or from another maturation pond. They should not receive untreated wastewater. Maturation ponds are aerobic for their whole depth because of their low concentration of organic material and their high concentration of algae. The ponds are intended mainly for pathogen removal. The pathogenic organism die off as the wastewater slowly passes through the maturation pond. The number of ponds required in the stabilization system depends on the required quality of the final effluent.

Pond Design Principles

Pre-Design Considerations

There are several factors which affect the size and layout of a pond system:

1.       The volume of sewage to be treated

2.       The strenght of the sewage to be treated

3.       The desired quantity of the final effluent from the pond system

4.       Climate

Before a pond system can be designed, values must be established for the first three factors.

The volume of sewage will depend first on the estimated population of the area to be served at the end of the design life of the system (which is usually assumed to be between 30 and 50 years), and secondly, the quantity of water each person is expected to use. This must take into account improvements in the water supply and higher standards of living, which will mean higher water consumption. It is usually assumed that 80 percent of domestically used water enters the sewers.

Figure2: The Symbiotic Relationship between Algae and Bacteria

The “strength” of sewage depends on its contents of organic materials, and is measured by it biochemical oxygen demand (BOD). This is the amount of oxygen that would be needed to oxidize the organic matter in the sewage and is usually measured in milligrams per liter (mg/l).

To estimate the strength of the sewage to be treated, the total amount of BOD contributed per person in the population served in considered. This will vary according to diet and how much food waste is dispostropical ed of through the sewers, but it most  tropical countries it will be roughly 40 g per person per day. The strength of sewage will be the total BOD per person divided by the water consumption per person.

However, where the effluent is used for trickle irrigation, the suspended solids contents will be important. Suggested values of BOD and FC forr various end uses for the effluent are given Table 1

Preliminary Treatment

Before sewage flows into the firs pond in the series, it may be screened to remove rags and other large objects that will float and cause problems in the ponds. The screens can be mechanic ally raked, but manually raked screens have the advantages that they can not break down and can be fabricated locally. The screenings should be buried or burned.

Table 1. Suggested Effluent Standars for Irrigationand Discharge

Method of Reuse	BOD (mg/l)	Fecal Coliforms Number/100 ml
Irrigation of trees, cotton, and other nonedible crops	60	50.000
Irrigation of citrus, fruit trees, folder crops, and nuts	45	10.000
Irrigation of deciduous, fruit trees, sugar cane, cooked vegetables , and sports playing fields	35	1.000
Discharge to a receiving stream	25	5.000
Unrestriched crop irrigation, including parks and lawns	25	100

In tropical countires, sewage tends to contain a lot of grit and sand which is washed into the sewers because people often use sand for cleaning kitchen utensils. The grit can be removed by passing the sewage along grit removal channels. If so, the grit should be regularly remove from these channels and buried. Alternatively, a sump can be constructed below the inlet to the first pond. The sump store the grit between desludging operations. The sump is easier to construction and maintain than the grit remove channel, and is therefore recommended.

Anaerobic Ponds

Anaerobic ponds are designed to receive BOD loadings of between 100and 400 gm/m³/d. The actual loading will depend on climate, with higher loading possible at higher temperatures. The retention time will usually be in the range of 1 to 2 days an a rate of BOD removal of up to 80 percent (but normally about 60 percent) is possible under tropical conditions.

Facultative Ponds

Facultative ponds are usually 1 to 1,5 meters  deep. If they are deeper than 1,5 m. the anaerobic layer of the ponds predominates. If the pond is less than 1 m deep, there will be problems with plants growing in the pond. Detention time varies from 5 – 30 days.

Maturation Ponds

 The number and size of maturationponds in a system depend on the bacteriological quality that is required of the effluent.

Construction

The layout of the various ponds  depends on the quality and topography of the land available. A sample layout is in figure 3.

Embankments should be simple and cheap to build and, where possible, rely on construction using homogenous soil with clay  or silt content. If the soils of the available site are not sufficiently impermeable, the embankments can be constructed with an impermeable clay blanket on the inside of the embankment.

Where the permeability of the soil is so high that losses due to infiltration are likely to exceed local evaporation raes, a lining should be used. This may be impermeable soil with high clay or silt content or plastic covered by a protective layer of soil.

At the water level, the embankments must be protected from erosion for a distance of 200 mm vertically above and below the water surface. Grouted rocks and stones are usually the simplest method for doing this, but pre-cast concrete slabs, if available, can be used.

The side slopes of the embankments will usually be about 1:3. With some soils , they may be steeper, but only if very thorough investigation confirm that this is feasible . (figure 4)

The embankment crest should be wide enough for all vehicles to drive to all parts of the pond system, and should be approximately 0,5 m above pond level.

Pond inlets to anaerobic and facultative ponds should discharge below survace to maintain anaerobic conditions in anaerobic ponds and to limit scum formation in facultative ponds. The inlets should discharge at least 10 m from the edge of the pond to reduce the possibility of sludge banks building up at the pond edge.

Interpond connection should be as simple as possible and should allow for flexible operation and expansion of the pond system. Anaerobic ponds should be provided with a means of being completely emptied for desludging.

Ponds inlets and outlets should be sited where they will keep effluent from traveling directly across the ponds. They are usually located in diagonally opposite corners.

Operation and Maintenance

The maintenance of waste – stabilization ponds is simple because they are a simple treatment system. A well maintained pond system will yield a good effluent quality with little labor.

Vegetation on embankments must be kept short bt cutting or moving because tall vegetation will shelter the ponds from wind. This will reduce the efficiency of the pond as wind is required to mix the layers of water  in the pond. Plants in the pond and near the water’s edge should be removed because they shade the ponds and encourage mosquito breeding. Lining part of the inner slopes with rocks of stones will help prevent aquatic plants from growing in shallowa water.

Scum mats may form on the pond survace because of gas bubbles which carry sludge particles to the survace to form mats of floating algae. These mats must be broken down (by water jet or long-handled rake) or removed. Scum removed from ponds should buried.

Three-Hour Practical

1.    List the three basic types of waste-stabilization ponds and describe the processes, principal functions, and maintenance requirements for each pond type.

Processes

Anaerobic ponds. Organic matter is removed by sedimentation and anaerobic digestion. This produces gas and soluble nutrients and leaves sludge.

Facultative pands. Organic matter is removed by sedimentation and aerobic treatment (upper layer) and anaerobic treatment (lower layer). Algae in the anerobic layer produce oxygen by photosynthesis and use nutrients and CO2 produced by bacteria. Bacteria use oxygen from the algae for breakdown of organic matter producing nutrients and carbon dioxide.

Maturation ponds. Bacteria break down wastes. Algae, through photosynthetic processes maintain an aerobic environment

Principal Function

Anaerobic ponds. Organic  matter removal and partial treatment.

Facultative pands. Pathogen destruction and organic matter treatment.

Maturation ponds.pathogen and suspended solids removal.

Maintenance Required

Anaerobic ponds. Require desludging when half-full of sludge (every 2 to 5 years); periodic vegetation and scum control.

Facultative pands. Require scum matter break-up or removal. Desludging done very infrequently. Periodic vegetation control along banks, as required.

Maturation ponds. Require periodic vegetation control.

2.    A pond system consisting of a facultative pond and maturation ponds is required to treat 100.000 m³/day of domestic sewage which has a BOD of 630 mg/l. The average temperature during the coolest season is 20^oC and the required bacterial quality of the effluent is less than 5.000 fecal coliforms/100 ml. Influent quality is 5 x 10⁷ fecal coliforms/100 ml.

Determine pond dimensions and retention times if facultative and maturation ponds are to be used.

Facultative pond

Area loading :

 mid-depth area :

 

assuming a depth of 1,5 m :

 

Maturation Pond

 

 For three maturation ponds with 3 days retention time

 

 Pound volume

assuming pond depth = 1,5 m;