--- title: "Pit privy; septic effluent disposal, rocky area" subject: "Sanitary Engineering" chapter: 9 key: "semester_6/sanitary_engineering:septic-tank-pit-privy-effluent-disposal" source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/septic-tank-pit-privy-effluent-disposal site: BCT Engineering --- # Pit privy; septic effluent disposal, rocky area Sanitary Engineering · Chapter 9: Disposal of Sewage from Isolated Buildings - Asked in: 2078 Chaitra, 8 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/septic-tank-pit-privy-effluent-disposal - Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-9 ## Question Describe in brief about the purpose and design criteria of pit privy. Also, with a neat sketch, discuss the suitable disposal of septic tank effluent for the rocks area with high ground water table. ## Short answer A pit privy contains and decomposes excreta to prevent disease; design: 0.04–0.06 m³/person/year, bottom ≥1.5 m above groundwater, lined pit, raised slab with squat hole. For rocky areas with high water table, use an evapotranspiration mound: effluent in perforated pipes over sand/gravel bed, planted, base ≥1 m above water table. ## Answer #### Pit privy: purpose A pit privy is a simple on-site latrine that holds human excreta in a pit, keeping it away from people, flies and water so that it decomposes safely, which prevents faecal-oral disease. It is used in rural and low-income areas without sewers or piped water. #### Design criteria - Location at least 15 m from a well/spring and 6 m from a house; pit bottom at least 1.5 m above the ground-water level. - Pit size: 0.06 m³/person/year for dry pits and 0.04 for wet pits, designed for 5-10 years; usually 1.0-1.2 m wide and 2-3 m deep, plus 0.5 m free space. - Pit lined with open-jointed brick/stone (or bamboo/wood), with the top 0.3 m watertight and raised above ground against surface runoff. - Slab of RCC/wood, 1.2 m × 1.2 m, with a 0.2-0.25 m squat hole and a cover lid. - Superstructure about 1.0 × 1.2 × 2.0 m with a door and roof; when the pit fills to 0.5 m below the slab, it is covered with soil and a new pit is dug. #### Disposal of septic tank effluent for a rocky area with a high water table Soak pits and trenches cannot be used, because the rock prevents absorption and the water table is high. The suitable method is an **evapotranspiration (ET) mound / sand mound**: 1. Effluent from the septic tank goes to a distribution box and perforated pipes (75-100 mm) laid in a gravel bed inside a mound of clean coarse sand, built on the rock or ground above the high water table. 2. The mound is 0.5-1.0 m high with side slopes of 1:3, and is covered with 0.3 m of topsoil and planted with grass or high-transpiring plants (banana, bamboo). 3. Effluent moves upward and through the sand by capillary action; most of it evaporates or is taken up by the plants, and the rest drains slowly. 4. Surface water is diverted away, and the base of the mound is kept at least 1 m above the water table. ```text plants / grass cover ___________________________ / topsoil + straw layer \ /-------------------------------\ / gravel + perforated pipe <- from \ / coarse sand bed dist.box ================================ rock / natural ground ^ at least 1 m GWT (water table) ``` Alternatively, a sand filter or constructed wetland can treat the effluent before it is released into a drain or stream. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the main purpose of a pit privy? A. To treat sewage to drinking standard B. To hold excreta in a pit away from people, flies and water so disease is prevented C. To produce biogas for cooking D. To collect rainwater Answer: B. It keeps excreta isolated so it decomposes safely, preventing faecal-oral disease. (Key point: Pit privy purpose) 2. Pit size is designed at A. 0.5 m3/person/year B. 0.006 m3/person/year C. 0.06 m3/person/year (dry) or 0.04 (wet), for 5-10 years, plus 0.5 m free space D. 1 m3 per household once Answer: C. The pit is usually 2-3 m deep and covered with soil and a new one dug when filled to 0.5 m below the slab. (Key point: Pit size) 3. Why are soak pits unsuitable for rocky ground with a high water table? A. They need too much sunlight B. They are too small C. They require chlorination D. Rock prevents absorption and the water table is high Answer: D. A soak pit needs deep permeable soil above the water table. (Key point: Unsuitable method) 4. How does an evapotranspiration mound dispose of effluent? A. Effluent rises by capillary action, and most is evaporated or taken up by plants B. It all drains to the water table C. It is burnt D. It is pumped into a river untreated Answer: A. Effluent in a gravel bed inside a sand mound is lost mainly through evaporation and transpiration; the mound stays at least 1 m above the water table. 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