In this series we are asking doomer optimists to give us a little taste of the kinds of actions they are taking in the face of the doom. The idea here is to start to map the breadth of the available activities out there that could get one started on a path toward something optimistic.
Just like our mini-manifestos, we all don’t have to agree about the best activities to pursue, or the ways in which we pursue them. What we agree on is that we should all be taking steps toward action.
It’s a coincidence that doomer optimism shortens to DO, but it could be our mantra, “do!”
Part 1: Small lot urban edible garden by Prasan (dhanuraashi@)
Part 2: How to start a micro-nursery for free by Sim
Part 3: Introducing Parents’ Nook by Parents Nook
Part 4: DIY decentralized water supply and treatment by Josh
Part 5: DO Perennials by Grant
Part 6: Growth Through Death by James Strand
Part 7: Productive Urban Landscapes by Roots Down
DIY Decentralized Water Supply and Treatment #2
Biological Slow Sand Filtration
By Josh Kearns
Water supply, quality, and treatment, including sanitation, are of major concern to all human settlements. This series examines critical topics in the provision of ample safe and healthy water for consumption, food preparation, washing and bathing, and resource recovery in sanitation. The focus is on small and medium scale technologies that are relatively low cost, utilize commonly available local materials, and can be implemented and operated by nearly anyone with a little training.
The large centralized drinking water and wastewater treatment facilities that most of us in affluent urban and suburban societies have taken for granted our whole lives are very costly to build and maintain, require large amounts of energy and material infrastructure used for example for pumping drinking water or sewage over vast distances, and are not likely to be long-term sustainable. Large municipal water systems are vulnerable to failure due to deferred maintenance, natural events such as hurricanes, negligence or criminality on the part of government or oversight agencies such as the Flint, Michigan water debacle, and interruptions in service caused by terrorist attacks or hackers. The coming years will see renewed interest in small and medium scale “off grid” treatment systems at the household and community level that can be operated with a high degree of local self-reliance. Likewise, in low-resource settings such as parts of the “developing world” decentralization and self-reliance in water supply, treatment, and sanitation form the only viable options for service provision.
Articles in this series will provide brief introduction to critical water topics, primers in the relevant scientific and engineering concepts for a general audience, overview and commentary on various supply and treatment technologies, and links and resources for more detailed investigation.
Special attention will be given to using biochar in water treatment applications for removal of harmful chemical substances, as this is my area of expertise. Interested readers are directed to my substack site where I am publishing in-progress chapters of my forthcoming book, A Field Guide to Biochar Water Treatment.
Issue #2 – A Quick Start Guide to Biological Slow-Sand Filtration
As noted in the previous installment in this series, concerns about the quality of water we consume as drinking water or use in food preparation fall into three broad categories: biological contaminants such as microbial pathogens that cause disease (e.g., giardia, E. coli), chemical toxicants that cause acute sickness or more commonly chronic illness (e.g., cancer, endocrine disruption), and aesthetic factors that may not be a health concern but cause water to be unappealing by taste, odor, and/or appearance.
Each type of water quality challenge requires a specific approach. That is to say, a treatment technology designed to kill or inactivate (prevent reproduction) of biological pathogens, such as chlorination, might not or likely will not also be effective for removing harmful chemical substances such as pesticides.
This issue provides a basic overview for using biological slow-sand filtration (BSSF) as a low-cost, scalable method for controlling harmful pathogenic microorganisms in drinking water.
The primary purpose of BSSF is to remove pathogenic microbes that cause intestinal disease (e.g., E. coli, parasites like giardia and cryptosporidium, and viruses). Secondarily, BSSF can also facilitate the biodegradation of some dissolved chemical pollutants. And, BSSF removes a portion of background dissolved organic matter, which, though benign from a health perspective, can cause problems for downstream treatment steps such as adsorption for removal of chemicals.
Here's a schematic for a BSSF unit constructed from a 55-gal (200 L) HDPE plastic drum that can supply up to 450 gallons (1,700 L) of treated water per day:
How it works
The purpose of the layers of coarse sand, pea gravel, and coarse gravel (the underdrain) is to keep the sand from exiting the contactor. Most of the treatment (pathogen removal) occurs in biofilm that develops in the top 0.5-2 cm of the sand, and to a lesser extent with depth in the sand bed. The biofilm is also called the schmutzedecke, German for “scum layer.”
The mechanisms of pathogen removal are:
· physical straining and sedimentation: pathogens get caught in biofilm
· adsorption: pathogens stick to surfaces of sand grains
· out-competition: biofilm microbes out-compete pathogens for food
· predation: biofilm protozoa eat pathogens
Pathogenic microbes that make us sick develop in the intestines of mammals – warm, dark, and anaerobic conditions. They occur in water sources through fecal contamination. Conditions in the BSSF – cooler, aerobic, full- or partially sun-lit – are not amenable for reproduction of intestinal bacteria. They do not survive at temperatures below 86 oF (30 oC) and there is insufficient organic material in the filter bed to feed them. They are also predated upon by protozoa.
Accordingly, BSSF is a long-established and reliable method for achieving high levels of pathogen removal. Excellent removal of pathogenic cysts and oocysts (e.g., giardia, crypto) occurs through physical straining and predation. Good but more variable (compared with larger microbes) removal of bacteria and viruses is achieved, sufficient to produce safe drinking water.
BSSF startup and maintenance
When a new BSSF unit is put into operation, it can take from several days to several weeks for the biofilm to fully develop (acclimate), depending upon environmental conditions. During this ripening period, it is advisable to utilize additional barriers to prevent pathogen exposure. This could include passing water through an already acclimated BSSF unit, or using UV light or chemical disinfectants such as chlorine to ensure drinking water safety.
BSSF maintenance is performed on an as-needed basis. Over time, sedimentation of particulates can occur in the biofilm and top few centimeters of sand, restricting flow through the filter. When the flow rate through the BSSF drops below the desired water throughput, the process of wet harrowing is used to remove excess sediment and restore adequate flow while minimally disturbing the biofilm.
In wet harrowing, the water above the top of the sand is stirred rapidly, causing sediments and particulates to resuspend in the water. This turbid (murky) water is the removed and discarded using a dipper or through a valve just above the sand. The idea is to suspend and remove accumulated sediments while leaving a patchy biofilm attached to sand grains in order to facilitate rapid reestablishment of a mature biofilm after maintenance. Pathogen removal rates decline somewhat after harrowing but can recover to baseline after a few days (warm filters recover faster than cold filters).
BSSF units can be operated continuously for years if properly maintained. It might be necessary from time-to-time to add a few centimeters of make-up sand to maintain desired bed depth, as some sand is unavoidably removed during wet harrowing.
BSSF design specifications
There are important guidelines to follow for setting up BSSF treatment units. The depth of the sand bed, the flow rate through the filter, the size range of the sand grains, and other factors are all important to the overall performance of a BSSF unit for producing ample, consistent, safe drinking water.
The detailed engineering principles behind successful BSSF are beyond the scope of this article. For a user-oriented, quantitative approach to BSSF design and operation for a variety of applications, see my (subscriber) substack post on this topic.
If you’d like a simple, instructional approach to setting up your own gravity-fed treatment system that includes (1) pre-treatment for removing excess sediment and particulates by gravel filtration, (2) BBSF treatment for pathogen control, and (3) biochar adsorption for removing chemical contaminants, graphical how-to manuals are available for free download:
· 80 gallon/day (300 liter/day) household treatment system
· 530 gallon/day (2,000 liter/day) small community treatment system
Treatment flow diagram for gravity fed 80 gal/day (300 L/day) system to produce safe, healthy, and pleasant drinking water. More information can be found at Aqueous Solutions.
Different options and configurations of gravel roughing filtration pre-treatment units are also available and will be the subject of future posts. Also, post-biochar treatment possibilities for providing additional barriers to pathogen exposure and addressing other water quality concerns will be described in future posts. If these are of interest, and in particular if you are interested in biochar adsorption for control of chemical water contaminants, consider subscribing to my substack book project site.