The minimalist resource utilization of biogas slurry from livestock farms

Solving the biogas slurry dilemma is all about hitting the right pressure point — using water to treat water.

What does “using water to treat water” mean?  How can biogas slurry be treated and utilized in a minimalist way?

Director Xu Shanxin, the inventor of soilless year-round water dropwort cultivation, illustrates this vividly in his poem:

No plowing, no harrowing, water as the field,

Vegetables float on the waves, roots anchor in the clouds.

Turn swords into ploughshares, set free the ammonia and nitrogen.

Mutual generation and restraint, cycling without form.

(Above: Qingyuan Qianrunda Environmental Company utilizes the foul pond of a pig farm to collect biogas slurry and directly grow soilless water dropwort, squeezing out the space previously occupied by water hyacinth — a single move to defeat the enemy)

As is well known, biogas slurry from livestock farms is a treasure, rich in nutrients. It not only contains the essential macronutrients for crop growth such as nitrogen, phosphorus, and potassium, but also various micronutrients like zinc and iron. The long-term anaerobic fermentation environment causes most pathogens, insect eggs, and weed seeds in the slurry to be suffocated to death, making it relatively safe. Biogas slurry is highly effective, with a high nutrient availability rate, allowing crops to quickly absorb and utilize it. Moreover, due to its high organic matter content, stable fertilizer efficiency, and long-lasting effects, it combines both quick and slow-release benefits.

However, due to the aging rural labor force and the short-sightedness of producers, few people use farmyard manure, and no one likes the black biogas slurry either. Even if they want to make use of it, the complicated methods and relatively heavy labor intensity are discouraging. As a result, a large amount of biogas slurry from livestock farms cannot be effectively treated, let alone resourcefully utilized.

(October 2018, Xu Shanxin, inventor of soilless year-round water dropwort cultivation, being interviewed by CCTV)

Currently, there are several ways biogas slurry is utilized in China:

1Soaking seeds in biogas slurry   Very low utilization rate

2Applying biogas slurry as topdressing Divided into foliar spraying and root irrigation, with low utilization

 3, Drip irrigation with biogas slurryVery low utilization rate.

Especially after autumn, the demand for biogas slurry in rural areas decreases even further. Yet livestock and humans continue to excrete waste every day.

To address the issue of livestock waste disposal, the state has invested a lot of funds, but these mainly target dry manure after solid-liquid separation. There are no better solutions for biogas slurry. Due to human ineptitude, biogas slurry — the best friend of farmers — is treated as pollution, as an enemy. Every year, huge sums are spent to destroy the nutritional value within it, a textbook case of robbing Peter to pay Paul. Both the state and enterprises are shouldering an unbearable burden.

Today, we introduce a convenient, foolproof method that can easily handle biogas slurry while also generating income and creating year-round landscapes.

This is the technology of soilless year-round water dropwort cultivation in natural water bodies, invented by the Anqing Mountain Spring Aquatic Vegetable Research Institute in Anhui Province.

Currently, the soilless water dropwort varieties developed by the institute have been selected over more than 10 years, and the soilless cultivation techniques have been tested in various climates and water bodies, making them quite mature. CCTV’s Science and Technology Garden program featured it twice in September and October 2018. The institute has established multiple demonstration bases in provinces such as Anhui, Shanxi, Guangdong, Jiangsu, and Chongqing, and has also developed various cultivation models tailored to the climate characteristics of northern and southern regions.

Xu Shanxin says that the so-called foolproof treatment and utilization method is simply to discharge the biogas slurry directly into a pond. As for the distance of the pond or the size of the water surface, everything can be adapted to local conditions. Then, the biogas slurry is fully blended with natural water and left to await planting. Of course, it’s best to apply some anti-seepage measures if needed.

The ideal water depth of the pond is above 1.5 meters, with no upper limit to the depth, and the area is unrestricted. However, for ease of management, a pond area of within 4 acres is preferable.

But then, what kind of carrier should be used?

Selection of floating beds.

Floating planting of water celery must have a carrier. The suitability of the carrier is extremely important. Below are several commonly used forms of planting in the market today.

 1Floating island planting

   Soilless water celery must be densely planted to achieve both yield and quality. The spacing between holes in floating islands is too large for dense planting, wasting a lot of space. Moreover, these floating islands are quite expensive, with high buoyancy, making them suitable for growing tall-stemmed crops like water bamboo and canna lilies, but not ideal for water celery or watercress.

2, Hole-punched foam board planting

Soilless planting with holes punched in foam boards has been one of the methods used both domestically and internationally for many years. However, it is not very suitable for growing water celery on ponds or for pollution control.

First, water celery is a low-light plant, meaning it doesn’t prefer strong light. Foam boards are white and reflect sunlight intensely under strong sunlight, completely altering the water celery’s preference for clustering and humid air. Forget about poor growth—even survival is a stroke of luck. Even if it survives, the growth will be stunted, the quality poor, with excessive coarse fiber, making it hard to eat, let alone profitable.

Second, the planting process is troublesome. Each seedling must be wrapped and fixed in a small hole using a sponge strip. How many holes need to be made per square meter? How many sponge strips need to be wrapped, and how many water celery plants need to be planted? How much area can one person plant in a day? With today’s labor shortages, the cost is no small figure.

Furthermore, apart from the areas with holes that allow ventilation, the rest of the surface is sealed off. The water surface is enclosed, so how can oxygen levels increase? How can there be vertical water circulation?

With the method described above, growing selenium-rich water celery as shown in the image below, or even aquaculture underneath, is almost impossible. Just keeping the plants alive is an achievement, but achieving the tenderness of vegetables and generating productive vitality is still a long way off.

(Under conditions of sufficient ammonia nitrogen and proper management, in the wild areas of the Yangtze River basin, such soilless water celery can yield over 20,000 pounds of net produce per mu.)

3, Bamboo strips,PVCand low-cost floating beds with fishing nets

Bamboo is very cheap, but labor costs are getting higher. Fishing nets are inexpensive, but they tend to loosen easily. Water celery may grow initially, but once it reaches over 20 centimeters, in the face of the ever-present wind in nature, it will sway uncontrollably—1. It can cause the roots of the water celery to be “sawed” and damaged; 2. Even if it grows taller, the quality will not be good. Over the years, people who have never grown water celery on the water’s surface have spent money without success and learned many hard lessons.

Currently, this type of floating bed is basically no longer in use.

Later, the switch was made to PVC pipes with fishing nets (as shown above), which was a step forward and looked much more aesthetically pleasing. However, cheap things are rarely good—the quality is poor, the fishing nets are elastic and loose, causing the water celery and other plants to shake and fall over easily.

4,The most practical and simple one currentlyFloating bed

After years of repeated experimentation and elimination, Xu Shanxin, the director of the Anqing City Mountain Spring Aquatic Vegetable Research Institute, and his team have developed several simple and practical floating beds. The method using plastic pipes with plastic mesh sheets and a cross frame underneath the mesh has become very popular.

The floating bed above is simple, practical, and has good breathability.

Xu Shanxin said, "The greatest truth lies in simplicity—not cheap simplicity, but the result of repeatedly refining and condensing complexity, just like smelting a pile of ore into a dagger. Only then can you achieve the unexpected and win with a single move."

In recent years, Teacher Xu Shanxin has traveled across the north and south, inside and outside the Great Wall. Based on the different climates of the north and south, he has designed floating beds with varying pipe diameters, materials, and hole sizes. One principle has always been upheld: oppose extravagant and costly approaches, pursue simple yet refined designs, maximize practicality, stay close to nature, and integrate environmental protection.

In the past two years, some farmers have also continuously tried to make their own floating beds. During production, many problems have gradually emerged. Some are very cheap to make but expensive to use. Others start off with decent plant growth but gradually lead to plant death, yielding no harvest and offering no pollution control benefits.

The floating bed is the land of soilless planting—never take it lightly.

Various methods of soilless planting

1,Planting on large bodies of water

Some large-scale farms produce a lot of biogas slurry and have vast water surfaces. The construction process is dirty and smelly—how can planting be carried out? Please see the construction site recorded in the photos.

After planting, the floating beds are towed by boat to suitable locations and fixed to tree stumps or wooden piles. Don’t worry about how they will grow—plants are living things. They will root and sprout on their own, absorbing rain and the abundant nutrients in the water.

They can also be planted directly near the ponds below the pigsty. Pig manure first enters a biogas digester or biogas bag, where it ferments for a certain period. A water pump is then used to transfer the biogas slurry into the pond, mixing it with the natural water body to address purification issues nearby. The harvested water celery is sold as premium produce, while the trimmings are fed to pigs or other herbivores nearby.

This truly provides an integrated, one-stop solution to the challenge of biogas slurry disposal.

Using water to manage water and applying biogas slurry in a foolproof way is actually a form of rustic refinement.

2，Planting in Small Ponds

   Connect the floating beds along the shore. Tie a rope to the first floating bed, and have one person pull it to the opposite side. Once the seedlings on that floating bed are evenly arranged, slowly tug it a bit, then plant the next one, and so on until the entire row is completed, then move on to the next row.

Pay attention to planting details: you can use either seedlings or older stems (the latter need to be cut into pieces about 20 centimeters long). Do not insert them into the net holes—simply scatter the water celery onto the floating bed like throwing rice seedlings, and they will take root and sprout.

No need to get into the water during planting. It’s very clean and convenient. Xu Shanxin has transformed what was once backbreaking labor in producing aquatic vegetables into a source of joy and a form of landscape art.

Once the plants reach a certain height, a suitable step can be built along the shore to pull the floating beds to land for harvesting while standing in the water. Alternatively, a floating raft can be made to paddle out and harvest from the water surface. In short, there’s no fixed method—it all depends on the local conditions.

Water celery requires ample water and nutrients, especially in summer. A lack of ammonia nitrogen will cause it to age quickly, resulting in excessive coarse fiber and making it hard to eat. Of course, pig farms have plenty of biogas slurry and fertilizer. But how to improve quality and how to harvest also involve certain techniques.

For more details, please consult the inventor, Teacher Xu Shanxin.

3Planting in Low-Lying Fields, Mountainous Areas, and Terraced Hills

Xu Shanxin says that water flows downhill—an eternal natural law. Therefore, for rural non-point source pollution, black and odorous water bodies, pig farm biogas slurry, and other wastewater, once they flow in naturally or are directed into low-lying fields or ponds, a certain amount of natural water is mixed in, the floating beds are placed on the water surface, and planting can begin directly.

As long as the ratio of biogas slurry to natural water is appropriate, it won’t damage the seedlings.

Below: Next to the water celery field is the biogas bag of Anhui Lüjian Breeding Pig Co., Ltd., which uses biogas for electricity generation and the biogas slurry for growing aquatic vegetables—nothing goes to waste. The company has made resourceful utilization of biogas slurry its second industry besides breeding, adopting a two-pronged approach.

As for whether harmful substances such as heavy metals in pig manure might be absorbed by water celery—that’s certainly a serious topic. Xu Shanxin’s team of soilless farming experts has already addressed it properly, with dedicated articles explaining it, which won’t be elaborated here.

So, how effective is soilless year-round water celery in absorbing biogas slurry?

Please refer to the experimental report from a pig farm in the Lüliang Mountain area of Shanxi Province—Shanxi Baishian Agriculture and Animal Husbandry Co., Ltd.

Note: To achieve the above purification results, it only took 30 days from the day the planting stems were sown.

Regarding the government’s emphasis on zero emissions, the soilless farming method fully adheres to it—these farms, because they can make money, are reluctant to discharge waste into rivers. They keep reusing it in cycles, and when the water becomes slightly clearer, new biogas slurry is added to further promote the production of high-quality water celery. Discharging the biogas slurry would be like letting economic benefits flow away.

Soilless aquatic crops grow entirely on the water surface, forcing the plants to rely on absorbing nutrients from the water to grow, requiring large amounts of N, P, and other nutrients. When these aquatic crops are harvested, the absorbed nutrients are removed from the water, thereby purifying it. They also provide attachment substrates and habitats for microbes and microorganisms, and their roots secrete substances that promote the growth of phosphorus- and nitrogen-loving bacteria, indirectly increasing the purification rate.

In summary, the real investment is in hardware like floating beds and pond excavation (floating beds can last over 10 years, and seedlings can be reused indefinitely under conditions of purification and rejuvenation), allowing the use of ponds and low-lying fields to gather surface runoff, accommodate biogas slurry, and employ soilless planting methods—turning water into fields, transforming wastewater into clean water, vegetables, and scenic beauty.

Although planting vegetation on water surfaces for pollution control in China has been around for years, it has mostly involved flowers and plants that stop growing once they reach a certain height. No one wants to harvest and regenerate them, so they eventually rot in the water. Even if frequent harvesting is desired, there’s no place to store the trimmings. Moreover, due to differing plant characteristics (some are biologically unsuitable for repeated harvesting), over-harvesting can actually inhibit plant growth. In contrast, year-round soilless water celery, when harvested at the right time, regrows like leeks after cutting—after 40 days, when it reaches 45–50 centimeters, it can be harvested again. This cycle repeats, accelerating the absorption of excess nutrients.

Director Xu Shanxin emphasizes that over the past decade, we’ve experimented with planting year-round water celery, watercress, water bamboo, lotus (seed lotus), rice, water chestnuts, and gorgon fruits. Ultimately, year-round water celery has proven the most effective:

1. High yield—under proper management, it can produce 30,000 pounds of vegetables per year, with large biomass and excellent pollution control effects.

2. High income—selling at 2 yuan per pound is also remarkable.

3. Unlike other plants that wither and regrow annually, it has no seasonality and remains evergreen year-round (at least in open-air planting south of the Huai River).

4. Low labor intensity, suitable for rural areas with severely aging labor forces.

Thus, underwater aquatic products, microorganisms in the water, and the green seedlings on the surface form a silent task force, each performing its role in joint combat, transforming excess nutrients in the water into a beautiful rural idyll. In this scenario, where is the need to burn money daily just to treat biogas slurry?

Xu Shanxin said that the great way must be extremely simple, but not simplistic in a cheap sense — rather, it is forged and condensed through countless trials. It is like sifting through sand to find gold.

This ultimate simplicity will eventually replace the utmost complexity of biogas slurry treatment.

(Author: Yang Chunhui)