Introduction

Wastewater and solid waste are produced every day by every household, market, hospital and industry. If they are not collected, treated and disposed of properly, they cause disease, bad smell, pollution of water and soil, and loss of property value.

Importance of management

• Public health: untreated sewage carries pathogens that cause cholera, typhoid, dysentery, hepatitis and diarrhoea. Heaps of garbage breed flies, rats and mosquitoes.
• Protection of water sources: it prevents pollution of rivers, lakes and groundwater that supply drinking water downstream.
• Environment and aesthetics: it removes foul odour, clogged drains and dumped refuse, and so keeps the city clean and attractive.
• Prevents flooding: blocked drains full of solid waste cause urban flooding (as seen in Kathmandu's rivers and drains).
• Economic benefit: treated water can be reused for irrigation, biogas and compost can be produced, and tourism and land value improve.
• Legal duty: the Local Government Operation Act and the Solid Waste Management Act, 2068 make local bodies responsible for the service.

Methods (components) of wastewater management

1. Collection: house connections, lateral and main sewers, appurtenances (manholes, inlets) carry sewage by gravity to one point. Systems are separate, combined or partially separate.
2. Treatment: preliminary (screens, grit chamber), primary (sedimentation), secondary (trickling filter, activated sludge, oxidation pond) and sometimes tertiary treatment.
3. Sludge treatment: thickening, digestion and drying beds.
4. Disposal: by dilution in a river or lake, or on land (irrigation, sewage farming). On-site systems (septic tank, pit latrine) serve areas without sewers.

Methods of solid waste management

Generation -> Storage -> Collection -> Transfer
   -> Transport -> Treatment/Resource recovery -> Disposal

1. Source reduction and segregation into organic, recyclable and residual waste.
2. Storage and collection in bins, door-to-door vehicles.
3. Transportation by tractors, tippers, compactors, often through a transfer station.
4. Treatment and recovery: composting, biogas, recycling.
5. Final disposal: sanitary landfill, incineration (for hospital and hazardous waste).

Sanitation is the science and practice of maintaining healthy living conditions by the safe collection, removal, treatment and disposal of human excreta, wastewater and solid waste, so that disease is prevented and the environment stays clean.

Main difference

The two systems differ in how night soil (human excreta) and other wastes are removed.

• Conservancy system (dry system): wastes are collected in separate dry containers or pits. Night soil is stored in pans or vaults and carried by manual scavengers or vehicles to a disposal site. Kitchen and bath waste (sullage) and storm water are taken through separate drains, and refuse by carts.
• Water-carriage system: all liquid wastes, including night soil, are carried by water through closed underground sewers by gravity to the treatment plant.

Comparison

Why water carriage is preferred

1. It is hygienic: excreta never come in contact with people, so there is little chance of disease.
2. There is no foul smell or fly breeding because the sewers are closed and underground.
3. It is automatic, fast and continuous; no scavengers are needed.
4. It takes less space and does not pollute streets or the groundwater near houses.
5. It can handle large quantities of waste from dense populations and allows proper treatment and reuse.
6. Its running cost is low, and it gives comfort and a better standard of living.

Its drawbacks are high initial cost, need for plenty of water and skilled operation, but for a populated town its benefits are much greater.

A sewerage system is the complete arrangement of sewers, appurtenances, pumping stations, treatment works and disposal works used to collect, convey, treat and dispose of wastewater and, when included, storm water from a community.

Types

• Separate system: one set of sewers for sanitary sewage and a different set (drains) for storm water.
• Combined system: one sewer carries both sewage and storm water.
• Partially separate system: one sewer carries sewage plus a part of storm water (roof and courtyard runoff), and the remaining storm water goes in surface drains.

Recommendation for a highly populated urban area

I recommend the separate system (with the storm drains laid as open or covered drains along roads).

Justification:

1. Rainfall in Nepal is intense and seasonal (monsoon). A combined sewer would be very large for rare peak flows, and would run almost empty in the dry season, with silting problems. In a separate system, the sewage sewer is small and always flows at reasonable velocity.
2. Treatment plant economy: only sewage reaches the plant, so the plant is smaller and works at nearly constant load. In a combined system the plant would be overloaded in rain or the overflow would carry raw sewage to rivers.
3. No overflow pollution: sewage is not discharged into rivers during storms.
4. Storm water needs no treatment and can be discharged directly into the nearest stream; its drains can be built cheaply and in stages.
5. Easier maintenance in narrow congested streets: smaller sewer pipes, easier to lay, and working conditions inside are better.
6. Pumping is cheaper where the sewage must be lifted.
7. Reuse: sewage of constant quality can be reused for irrigation or biogas.

The only disadvantage is higher total cost because two systems are laid, and illegal connection of roof drains into sewers must be controlled. For a dense urban area with heavy monsoon rain, such as Kathmandu valley, the separate system is more suitable.

Comparison of the three systems

Separate system

Merits: smaller sewers; treatment plant is smaller and cheaper; no storm overflow pollution; storm water can go directly to streams; sewers flow at good velocity. Demerits: two systems cost more; small sewers can block easily; storm drains may get contaminated by wrong connections. Favourable conditions: heavy and long monsoon rain; dry-season sewage flow very small compared with the storm flow; flat terrain with little fall; a natural stream is near for storm water; sewage must be pumped.

Combined system

Merits: one pipe laid, so a single house connection and cheaper total cost; large sewers are self-flushed by storm water and are easy to enter for cleaning; only one system to maintain in narrow streets. Demerits: large sewers are costly; the treatment plant must handle the large wet-weather flow or overflow pollutes the river; flow is low and silting occurs in dry weather; pumping is costly. Favourable conditions: moderate, well-distributed rainfall; limited space for two sets of pipes; steep ground so that dry flow keeps sufficient velocity; storm drainage and sewage both needed at the same time as in a densely built-up area with an existing disposal point.

Partially separate system

Merits: moderate size of sewer; economical compared with the separate system; roof and courtyard runoff flushes the sewer. Demerits: the sewer size is still larger than in the separate system; the plant sees some storm load; household connections are complicated, and silting may occur in the dry season. Favourable conditions: when part of the storm water (from roofs and paved yards) can be admitted but street runoff must be carried separately, as in areas with moderate rainfall and where an existing storm drain exists.