Part of the current construction at the hospital that we're doing is connecting piping for a gravity-powered sewage system. We've built three new showers and three new latrines to increase the capacity of our malaria ward. It's an 80-bed ward, and the MSF standard is one latrine per 20 people and one shower for 40 people, with a latrine and shower reserved for staff use. We used to have two latrines for patients and one for staff, with one shower each for patients and staff, so we've expanded the capacity to five latrines/four showers for patients, keeping the 1/1 for staff. For the plans for the evacuation of the showers, we're sending the waste-water to the same treatment center as the kitchen's waste-water. It's a run of about 30 meters, slightly downhill.
The complicated part is the slightly downhill part. We have to assure a slope of between 1% and 2% for the evacuation pipe. That's simple math to find out how much lower the end has to be from the beginning, as a function of the pipe's length, but the tough part is measuring from a reference point. We don't really have any surveying material (plus I'm not really sure how to use surveying material; you've seen the guys with the orange vests ruining your day by stopping one lane of traffic--I wish they were here now), so we've got to find another way.
We could, I imagine, set up a string between two pickets and use a builder's level to get it straight, but over 30 meters, with drooping string and imprecise levels, it's not ideal. Instead, we'll go for a trick that I learned in the training last month (10 points to MSF for teaching useful techniques). We'll take a tube (ideally transparent, but we don't have transparent tubing) and add water. Give it a second or two to overcome resistance (remember the concept of coefficient of kinetic friction from high school physics?), and the water in the tube at each end is the exact same level. That's our reference point. Magic.
We put one end of the hose at the beginning of our 30 meter run, and stretch the hose out to the end of the run. We fill it with water, and rapidly find all the leaks in the hose. That's not helpful.
10 minutes of patching leaks later, we start again.
We attach the downhill side to a picket, and top up the hose with water. It's a slow process of watching the water level rise, then slowly sink as it levels itself across 30 meters, adding more water and having the water overflow, letting it level out, one end raising as someone scratches their nose and all the water dumps out the other end, losing the level of water since our tube isn't transparent, running to the faucet for more water, finding that last little leak, and finally getting a stable level.
All of the bricklayers and assistants have stopped what they're doing to gather around and stare. I explain the concept of what we're doing; I feel like a magician. They all grasp the concept quickly, and jump in to help.
We find the reference point, and it's not as high as we had hoped. We have to dig down about 10 more centimeters to make the slope, and we'll have to assure that the slope is even throughout the run of the pipe. Every centimeter we lose in depth is translated into a loss in working volume of our grease trap that will filter the wastewater before we infiltrate it back into the soil. But we can't let sewage stagnate in the pipe, so depth shall be lost. We settle on the final level, and I leave the team to finalize the pipe run.
I head back to the office to follow up on leads on additives for waterproofing cement, to seal our grease traps to make them effective. That's next week's project.