How Town Water and Sewer Work

• Contact time: at least 30 minutes at 64 °F (18 °C) or warmer, longer in cold water, so chlorine goes in as water enters a tank or a long pipe (WHO/WEDC, p. 43; water chlorination).
• Residual is the chlorine left over; it shows the germs are dead and guards the pipes. Aim for 0.2–0.5 mg/L at the tap, higher near the plant, lower at the far ends (WHO/WEDC, p. 46; Sphere, p. 135). Many people complain at 0.6–1.0 (CAWST Testing, p. 214) and go back to better-tasting, unsafe sources (Sphere, p. 136).
• Clear water first: chlorine misses germs hidden in silt and is weak against Cryptosporidium (WHO/WEDC, p. 43). Get cloudiness below 5 turbidity units first (Sphere, p. 136).

All pipes leak. When the pressure drops, dirty water enters through the breaks, more than any chlorine people will drink can deal with, so a supply that keeps going off must be treated as contaminated and disinfected where it is used (WHO/WEDC, p. 46; backflow). That is why towns put out a boil-water notice after a main break or a long power cut.

Cross-connections: water can siphon backwards through a hose (OSU Twelve Simple Things, p. 4), so a hose lying in a bucket or stock tank can be sucked into the pipes when pressure drops. Fit backflow preventers (Texas, p. 48; backflow preventer) and keep an air gap between a hose and what it fills (OSU EM9273, p. 2). Mixed drinking and sewage plumbing has caused outbreaks (Ship's Medicine Chest, p. 95).

At home, when mains break or the water is declared unsafe:

1. Shut the house valve, or cracked lines may pollute your supply and gravity drain the water heater and toilet tanks (Map Your Neighborhood, p. 23; OSU Water, p. 3).
2. Use the water in the house: open the highest tap to let air in and draw from the lowest; drain the water heater with its power or gas off (FEMA, p. 40).
3. Treat tap water until it is declared safe (Water Purification guide).

• Water from above town needs no pump; springs and gravity supplies come first (Sphere, p. 131).
• Water from below town is pumped up once: bring generators to the pumps (WHO/WEDC, p. 24), run them a few hours to fill the tanks, and let gravity deliver ("Pump into storage" and ram pumps in the Water Wells guide).
• If the pipes can't hold pressure, run one sound main to standpipes: one tap per 250 people, no home farther than 500 m (550 yd), no queue over 30 minutes (Sphere, p. 131).

A slow sand filter is the best single treatment a village can build and run: local materials, no chemicals, a caretaker trained for a short time (IRC, pp. 19–20). Water trickles through fine sand at 0.1–0.3 m (4–12 in) an hour; a slimy living skin, the schmutzdecke, grows on top and eats germs (p. 19). A mature filter removes 95–100% of fecal bacteria and nearly all viruses and cysts (p. 38).

• The box (IRC, pp. 31–34; drawn with every valve, p. 32): watertight concrete, ferrocement, stone or brick, 2.5–3 m (8–10 ft) tall; 1–1.5 m of raw water over 0.8–0.9 m of fine sand (grains 0.15–0.30 mm, no clay or soil; never let it get below 0.5 m), over 0.3–0.5 m of graded gravel and perforated drains. An outlet weir just above the sand keeps it under water.

The box, its depths and the living skin on top of the sand (IRC, pp. 19–20, 31–34; a diagram drawn for this guide).

• Size: 0.02–0.08 m² (¼ to 1 sq ft) of sand per person (p. 20), so 20–80 m² (200–900 sq ft) for 1,000 people (arithmetic); a person with one tap uses about 50 litres (13 gallons) a day (p. 46). Build two or more beds, run at 0.1–0.2 m an hour; one may go to 0.3 for a day or two while its partner is cleaned (p. 40).
• Not for muddy water: over 50 turbidity units for weeks, or 100 for days, clogs it. Settle the water first in a pond or gravel roughing filter, or draw it through the riverbed (p. 19).

Running and cleaning it (EPA Slow Sand Filtration, pp. 9–12):

• Run it steadily, day and night; change the flow by no more than half in a day and never switch it on and off, or the living layer starves of oxygen. Keep water above the sand. Below 41 °F (5 °C) filter more slowly, and don't clean it in winter.
• Clean when the water reaches the overflow or the flow falls too low: drain to a few inches below the sand, scrape off half an inch or less with flat shovels, and keep off the uncleaned parts (pp. 10–11). Re-sand before the bed is under 24 inches (p. 11), usually in the second or third year (IRC, p. 34).
• Refill from the bottom, then run it to waste at least a day (an hour for each hour it was off) until tests are good (EPA, p. 12). A new bed takes weeks to months to ripen (p. 7).
• Never chlorinate before the filter: it kills the living layer. Chlorinate what comes out (WHO/WEDC, p. 46).

A homestead-sized filter is in the Water Purification guide.

Drops per gallon and the pool-shock stock solution are in "Step 3 — Kill the germs" in the Water Purification guide. For a town tank:

• Dose where the water has 30 minutes before the first tap, as it enters the clear-water tank or a long main (WHO/WEDC, p. 43).
• Use bleach or calcium hypochlorite (pool shock), not chlorine gas: pure chlorine is "difficult and dangerous to use" (VITA, p. 133). Make powder into a paste before adding it (WHO/WEDC, p. 46). VITA's table gives amounts per 1,000 gallons: very clear water starts at 5 parts per million, slightly hazy water at 10 (VITA, p. 134).
• A drip feeder with no power: barrels on a shelf feed weak bleach and raw water side by side at the same steady pressure into the town's tank. Start with ½ pint of bleach in 50 gallons of water for each 500 gallons treated, and adjust until the tank water has a noticeable chlorine taste (VITA, pp. 137–139; the barrels, p. 138).

Testing the residual (WHO/WEDC, pp. 44–45; the steps drawn, p. 45): a DPD tablet turns a sample red, and you match it against a color chart (stock tablets now (rule of thumb)). Test where the chlorine goes in, at the nearest house and at the far ends of town; daily for a new or repaired system, then at least weekly. With no kit, a faint chlorine smell after 30 minutes means enough was added (Water Purification guide). If the supply keeps going off, chlorinating the pipes is pointless: have people disinfect at home (WHO/WEDC, p. 46), and keep scarce chemicals for places like the hospital and schools (p. 26).

WHO's steps for a small town's broken pipes (WHO/WEDC, pp. 15–18):

1. Map the damage with the staff who know the system and its drawings; walk the network and mark every break.
2. Tell people what is off, what you're doing and when, and how to stay safe.
3. Give them other water if repairs will take more than a few hours: tankers, and advice on local springs and wells for washing.
4. Isolate the break with the valves, so the rest of town keeps water. If the valves can't be found, fit new ones.
5. Repair from the source outward, 500–1,000 m (⅓–⅔ mile) between valves at a time, with repair clamps or a new pipe laid on the ground for now. Shore the trench, keep traffic away, and find other buried lines first (shoring, a clamp and backfill, p. 17).
6. Test, flush and disinfect: hold 1½ times normal pressure for 4 hours and watch the joints; flush until clean; fill with water dosed at 80 g of calcium hypochlorite per 1,000 litres (about 11 oz per 1,000 gallons (arithmetic)) for 24 hours; flush until the strong chlorine smell is gone (pp. 17–18).

Hidden leaks (rule of thumb): a wet or green strip in dry weather, water in a valve box, a hiss heard through a steel rod on a valve at night, a tank that drops at night when nobody uses water. People help find leaks if you ask (WHO/WEDC, p. 26).

Keeping the water clean is the cheapest treatment: manage excreta, garbage and storm runoff, control erosion and farm runoff, and keep people out of the source; getting the town's sewage under control may matter more than repairing the water plant (WHO/WEDC, p. 24; what to keep away, same page). Walk the source regularly and check drainage, fences, latrines and garbage, and test any new source for arsenic and fluoride as well as germs (Sphere, pp. 135–136). Latrines and graves stay 100 ft from water (Sanitation guide); fence the intake, filters and tanks and lock the hatches (rule of thumb).

Tell people what is affected, what you are doing and when, and what they should do, and keep the updates coming (WHO/WEDC, p. 15); news eases worry, cuts waste and gets leaks reported (p. 26). Secrecy fails: Montreal's silence didn't stop hoarding, and boil-water warnings meant nothing to people with no way to boil (EMP Commission, pp. 160–161). A notice that works (rule of thumb): is the water safe, boil, or off; where the nearest safe water is; don't flush if the sewer is down; when the next notice comes. Post it in the same places daily and send it by runner ("Runners, relays and written messages" in the Signals & Morse guide).

A sanitary sewer is a branching network of pipes running downhill to the plant (Compendium, pp. 92–93; drawn, p. 92):

• It slopes all the way, fast enough to scour itself: about 2 ft (0.6–0.7 m) a second. Where the slope can't be kept, a pumping station lifts the sewage. Main sewers lie 5–10 ft (1.5–3 m) under the roads.
• Manholes sit at set distances and at every junction and bend, for inspection and cleaning.
• Sewers need water: keeping that speed takes over 100 litres (26 gallons) a person a day. With water rationed, solids settle and pipes block (rule of thumb).
• Grease, rags and grit block them; crews rod, flush, jet and bail them. Sewers "can be dangerous because of toxic gases" and are work for trained crews (p. 93).

A lift station (pumping station) is a deep pit, the wet well, where sewage collects until pumps lift it to a higher sewer (Compendium, p. 92). When its power fails, the wet well and the sewers behind it fill, sewage comes up the lowest drains first (basement floor drains, low toilets and showers), and then out of manholes into streets and creeks (rule of thumb) (sewer overflow). A generator or engine pump at each key lift station keeps the sewer working (rule of thumb). Nobody goes down into the wet well ("Deadly dangers", below).

A gravity sewer and its lift station, working and with the power off (Compendium, pp. 92–93; OSU Sanitation, p. 5; a diagram drawn for this guide).

A plant cleans sewage in stages (EMP Commission, p. 158; sewage treatment):

Lagoons (waste stabilization ponds) need no electricity: three or more ponds in a row, a deep anaerobic pond (2–5 m, 1–7 days) that settles and digests the solids, a facultative pond (1–2.5 m, 5–30 days), and shallow maturation ponds (0.5–1.5 m) where sunlight kills germs. They cost little to run and take sudden loads, but need a lot of land. Fence them, skim the scum, pull water plants that breed mosquitoes, and dig out the first pond every 2–5 years, when it is a third full of sludge (Compendium, pp. 110–111).

Cave Junction's sewage plant treats and disinfects the town's wastewater to the limits in its discharge permit (county plan, p. 108). It was on the county's list of places to study for a solar-and-battery microgrid (p. 288). The county warns that sewage plants and lift stations stop when the power fails, with overflows, and that treatment plants often sit low in floodplains (p. 286); the city rates its chance of a river flood as high (p. 133).

Sludge is the solids a plant collects; raw sludge goes to a digester, where bacteria break it down without air (EMP Commission, p. 158; anaerobic digestion). On drying beds of sand and gravel over drains, 50–80% of the liquid drains or dries off and the cake can be shoveled out after 10–15 days; roof them in a wet climate. Dried sludge is not safe yet, and the water draining off needs treating too (Compendium, pp. 118–119). "Even after treatment, sludge is rarely pathogen-free": wear boots, gloves and a mask, and follow the rules on which crops may get it (p. 139); keep it off anything eaten raw (rule of thumb).

Storm drains carry rain from the streets straight to a river or lake. No toilet may be connected to one; clear them of trash before the rainy season (Compendium, pp. 94–95). Combined sewers carry rain and sewage in one pipe and overflow on purpose in storms so the plant isn't swamped; they are no longer thought good practice (p. 93). Rain gets into separate sewers too, through manholes and cracked pipes (pp. 92–93), so a sewer near its limit backs up in heavy rain (rule of thumb).